This case can't run when IMGPROTO=nbd, since it needs to create some
internal snapshot which would fail for EOF write request, even when
TEST_IMG is exported with "-f raw" in common.rc, so set _supported_proto
to file.

_require_command() is changed to tip what util is missing, instead
of printing a blank.
Signed-off-by: NWenchao Xia <xiawenc@linux.vnet.ibm.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>