block: Honor blk_set_aio_context() context requirements

block: Honor blk_set_aio_context() context requirements

The documentation for bdrv_set_aio_context_ignore() states this:

 * The caller must own the AioContext lock for the old AioContext of bs, but it
 * must not own the AioContext lock for new_context (unless new_context is the
 * same as the current context of bs).

As blk_set_aio_context() makes use of this function, this rule also
applies to it.

Fix all occurrences where this rule wasn't honored.

Suggested-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Sergio Lopez <slp@redhat.com>
Message-Id: <20201214170519.223781-2-slp@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>