um: virtio_uml: query the number of vqs if supported

When the VHOST_USER_PROTOCOL_F_MQ protocol feature flag is set, we can
query the maximum number of virtual queues. Do so when supported and
extend the check to verify that we are not trying to allocate more
queues.

Signed-off-by: Benjamin Berg <benjamin.berg@intel.com>
Link: https://patch.msgid.link/20241103212854.1436046-5-benjamin@sipsolutions.net


[add a message to the WARN_ON]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>