We have been checking whether qemu-img supports the -o compat
option by scraping the -help output.

Since we require QEMU 1.5.0 now and this option was introduced in 1.1,
assume we support it and ditch the help parsing code along with the
extra qemu-img invocation.
Signed-off-by: NJán Tomko <jtomko@redhat.com>
Reviewed-by: NJohn Ferlan <jferlan@redhat.com>