iotests: 181 does not work for all formats

Test 181 only works for formats which support live migration (naturally,
as it is a live migration test). Disable it for all formats which do
not.
Signed-off-by: NMax Reitz <mreitz@redhat.com>
Message-id: 20170621131157.16584-1-mreitz@redhat.com
Reviewed-by: NJohn Snow <jsnow@redhat.com>
Signed-off-by: NMax Reitz <mreitz@redhat.com>

tests/qemu-iotests/181

@@ -43,6 +43,8 @@ trap "_cleanup; exit \$status" 0 1 2 3 15
 . ./common.qemu
 
 _supported_fmt generic
+# Formats that do not support live migration
+_unsupported_fmt qcow vdi vhdx vmdk vpc vvfat
 _supported_proto generic
 _supported_os Linux