vol_create: Update specific error msg for skip cases

As test on rhel7 with rhev qemu, the error msg is different from
upstream qemu, so update the error msg which from qemu-img to
skip unsupported img type cases.

As commit '790dfee' added the metadata preallocation support in
libvirt 1.0.0, skip preallocation error case on libvirt before
1.0.0.

Signed-off-by: Wayne Sun <gsun@redhat.com>
Merge pull request #133 from waynesun09/vol_create

vol_create: Update qemu-img error msg for skip unsupported cases