block/export: only acquire AioContext once for vhost_user_server_stop()

block/export: only acquire AioContext once for vhost_user_server_stop()

vhost_user_server_stop() uses AIO_WAIT_WHILE(). AIO_WAIT_WHILE()
requires that AioContext is only acquired once.

Since blk_exp_request_shutdown() already acquires the AioContext it
shouldn't be acquired again in vhost_user_server_stop().

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20230323145853.1345527-1-stefanha@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>