Add VHOST_USER_PROTOCOL_F_CONFIGURE_MEM_SLOTS

Add VHOST_USER_PROTOCOL_F_CONFIGURE_MEM_SLOTS

This change introduces a new feature to the vhost-user protocol allowing
a backend device to specify the maximum number of ram slots it supports.

At this point, the value returned by the backend will be capped at the
maximum number of ram slots which can be supported by vhost-user, which
is currently set to 8 because of underlying protocol limitations.

The returned value will be stored inside the VhostUserState struct so
that on device reconnect we can verify that the ram slot limitation
has not decreased since the last time the device connected.

Signed-off-by: Raphael Norwitz <raphael.norwitz@nutanix.com>
Signed-off-by: Peter Turschmid <peter.turschm@nutanix.com>
Message-Id: <1588533678-23450-4-git-send-email-raphael.norwitz@nutanix.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>