Handle non-script grub2-install

grub2-install has been re-written [1] so isn't always a shell script,
so just exec it directly.  Also use $() as that follows the rest of
the file.

[1] http://git.savannah.gnu.org/cgit/grub.git/commit?id=cd46aa6cefabd7b0fcb15cd614577e1ab8c7a841

Change-Id: Ib4fd7c6854fb485c0da6a79babf4a3d45a8a0f2c

elements/vm/finalise.d/51-bootloader

@@ -42,9 +42,9 @@ function install_grub2 {
 
     # XXX: grub-probe on the nbd0/loop0 device returns nothing - workaround, manually
     # specify modules. https://bugs.launchpad.net/ubuntu/+source/grub2/+bug/1073731
-    GRUBNAME=`which grub-install` || echo "trying grub2-install"
+    GRUBNAME=$(which grub-install) || echo "trying grub2-install"
     if [ -z "$GRUBNAME" ]; then
-        GRUBNAME="bash -x `which grub2-install`"
+        GRUBNAME=$(which grub2-install)
     fi
 
     # If no GRUB2 is found, fallback to extlinux