hmp: Add nbd_server_remove to mirror QMP command

Since everything else about the nbd-server-* QMP commands is
accessible from HMP, we might as well make removing an export
available as well.  For now, I went with a bool flag rather
than a mode string for choosing between safe (default) and
hard modes.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20180125144557.25502-1-eblake@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>