Handle modern sfdisk and correctly align image partition

As described in the comments, sfdisk was rewritten for util-linux 2.26
(as shipped in F22) and now interprets arguments a sectors, rather
than cylinders.

The current partitioning line is "1 - - *" (start/size/type/bootable)
which means you start getting:

---
/usr/sbin/grub2-install: warning: this msdos-style partition label has
no post-MBR gap; embedding won't be possible.

/usr/sbin/grub2-install: warning: Embedding is not possible.  GRUB can
only be installed in this setup by using blocklists.  However,
blocklists are UNRELIABLE and their use is discoura ged..

/usr/sbin/grub2-install: error: will not proceed with blocklists.
---

when building images, because the start is interpreted by the new
sfdisk as sector 1 and it crams the partition right next to the MBR.

Specifying "-" for the size is undefined in the man page; even reading
the source it's not totally clear what "-" for the size does [2].  In
any case, the alignment is wrong in sectors or cylinders; we want to
be a multiple of 4KiB for best performance.

The intent here is to create one single, Linux, bootable, partition
taking up the whole disk starting at 1MiB, so "2048 + L *" makes this
clear.

We use the -uS argument to ensure both versions treat this start-value
as a sector offset (newer sfdisk essentially ignores the argument).
As described in the comments, bugs in the older sfdisk necessitate
usage of "--force".

Although we could choose more or less, it seems most common to align
to a 1MiB boundary (i.e. starting at sector 2048).  libguestfs has
some disucssion around --alignment and where it sets it's default to
this [3].  The 2.26-era sfdisk also defaults to putting partitions
here.  1MiB should be enough for GPT schemes in the future as well.

[1] https://github.com/karelzak/util-linux/blob/master/libfdisk/src/script.c#L1050
[2] https://bugzilla.redhat.com/show_bug.cgi?id=1249893
[3] http://libguestfs.org/virt-resize.1.html

Change-Id: I2c2966f98d1d5ad4ebb433ea148b3b26c65dc1b5

elements/vm/block-device.d/10-partition

@@ -17,10 +17,26 @@ if [[ "$ARCH" =~ "ppc" ]] ; then
 ;
 EOF
 else
-# Create 1 partition far enough up the disk to permit grub to be installed on
+    # sfdisk changed around 2.26 and was largely rewritten.  The old
-# the MBR.
+    # version defaults to c/h/s and interprets arguments as cylinders.
-    sudo sfdisk $IMAGE_BLOCK_DEVICE << EOF
+    # The new version defaults to sectors.
-1 - - *
+    #
+    # Luckily, the old version has a -uS flag (ignored by the new
+    # version) to interpret arguments as sectors.  However, it
+    # incorrectly calculates the c/h/s offset when finding free space,
+    # and gets upset [1], requiring the use of --force.
+    #
+    # TODO: switch this to parted, or something saner (is there such a
+    # thing when talking about partitioning?)
+    #
+    # The below command creates a partition that starts at 1MiB --
+    # which is the standard place to start a partition these days --
+    # and fills up the disk.  The zeros are to ignore the other 3
+    # primary partitions (probably not needed, but left for safety).
+    #
+    # [1] https://bugs.launchpad.net/ubuntu/+source/util-linux/+bug/1481158
+    sudo sfdisk -uS --force $IMAGE_BLOCK_DEVICE << EOF
+2048 + L *
 0 0;
 0 0;
 0 0;