Disk-image-create should allow sending compat flags to qemu-img

Add a --options flag that allows disk-image-create to send custom
options to qemu-img calls. It is useful for forcing compat into
qemu-img created images.

Usage:
    disk-image-create --qemu-img-options compat=0.10 base ubuntu

Change-Id: I73ee2c86abc115220d51ffebbbe9ce3c4d92f188
Closes-Bug: #1329746
Merge "Disk-image-create should allow sending compat flags to qemu-img"