iotests: change supports_quorum to use qemu_img

iotests: change supports_quorum to use qemu_img

Similar to other recent changes: use the qemu_img() invocation that
supports throwing loud, nasty exceptions when it fails for surprising
reasons.

(Why would "--help" ever fail? I don't know, but eliminating *all* calls
to qemu-img that do not go through qemu_img() is my goal, so
qemu_img_pipe() has to be removed.)

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Hanna Reitz <hreitz@redhat.com>
Message-Id: <20220321201618.903471-12-jsnow@redhat.com>
Signed-off-by: Hanna Reitz <hreitz@redhat.com>