vhost-user-blk-server: notify client about disk resize

vhost-user-blk-server: notify client about disk resize

Currently block_resize qmp command is simply ignored by vhost-user-blk
export. So, the block-node is successfully resized, but virtio config
is unchanged and guest doesn't see that disk is resized.

Let's handle the resize by modifying the config and notifying the guest
appropriately.

After this comment, lsblk in linux guest with attached
vhost-user-blk-pci device shows new size immediately after block_resize
QMP command on vhost-user exported block node.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Message-Id: <20230321201323.3695923-1-vsementsov@yandex-team.ru>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>