um: virtio: Implement VHOST_USER_PROTOCOL_F_SLAVE_REQ

Implement the communication channel for the device to notify
us of some events, and notably implement the handling of the
config updates needed for the combination of this feature
and VHOST_USER_PROTOCOL_F_CONFIG.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Richard Weinberger <richard@nod.at>