block: Make wait/mark serialising requests public

Make both bdrv_mark_request_serialising() and
bdrv_wait_serialising_requests() public so they can be used from block
drivers.

Cc: qemu-stable@nongnu.org
Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 20191101152510.11719-2-mreitz@redhat.com
Signed-off-by: Max Reitz <mreitz@redhat.com>
(cherry picked from commit 304d9d7f)
 Conflicts:
	block/io.c
*drop context dependency on 1acc3466
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>