8 MiB is not enough when using the grub2 element with centos7 images,
which installs binaries from the shim-x64 and grub2-efi-x64 packages
under /boot/efi. 12 MiB ought to be enough for anybody.
Change-Id: I8594d1fe05242f246a5809740a115ab2f84ac5a3
This moves the block-device default out of the "vm" element and into a
selection of other elements. There's "mbr" which retains the status
quo. There's an EFI version that has the boot/grub partitions as
required. In between there's the GPT only version, which is useful
for architectures like power without EFI, but still want possible
larger disks using GPT.
Change-Id: I4a566a97d073fc0dda0ab2494ac988fe015800a9