svirt_attach_disk: Modify rawio capabilitiy check with bit and

Run capsh --decode on host will met problem on rhel6, so change
to use 'bit and' with rawio capabilitiy value.

Bug on rhel6:
  $ capsh --decode=fffffffc00020000
will got endless output in stdout

Signed-off-by: Wayne Sun <gsun@redhat.com>