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
