We have a few bdrv_*() functions that can either spawn a new coroutine
and wait for it with BDRV_POLL_WHILE() or use a fastpath if they are
alreeady running in a coroutine. All of them duplicate basically the
same code.

Factor the common code into a new function bdrv_run_co().

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-id: 20200520144901.16589-1-vsementsov@virtuozzo.com
   [Factor out bdrv_run_co_entry too]
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>