Lift max memory slots limit imposed by vhost-user

Historically, sending all memory regions to vhost-user backends in a
single message imposed a limitation on the number of times memory
could be hot-added to a VM with a vhost-user device. Now that backends
which support the VHOST_USER_PROTOCOL_F_CONFIGURE_SLOTS send memory
regions individually, we no longer need to impose this limitation on
devices which support this feature.

With this change, VMs with a vhost-user device which supports the
VHOST_USER_PROTOCOL_F_CONFIGURE_MEM_SLOTS can support a configurable
number of memory slots, up to the maximum allowed by the target
platform.

Existing backends which do not support
VHOST_USER_PROTOCOL_F_CONFIGURE_MEM_SLOTS are unaffected.

Signed-off-by: Raphael Norwitz <raphael.norwitz@nutanix.com>
Signed-off-by: Peter Turschmid <peter.turschm@nutanix.com>
Suggested-by: Mike Cui <cui@nutanix.com>
Message-Id: <1588533678-23450-6-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>