virtio-blk: add missing AioContext lock

virtio-blk: add missing AioContext lock

virtio_blk_update_config() calls blk_get_geometry and blk_getlength,
and both functions eventually end up calling bdrv_poll_co when not
running in a coroutine:
- blk_getlength is a co_wrapper_mixed function
- blk_get_geometry calls bdrv_get_geometry -> bdrv_nb_sectors, a
  co_wrapper_mixed function too

Since we are not running in a coroutine, we need to take s->blk
AioContext lock, otherwise bdrv_poll_co will inevitably call
AIO_WAIT_WHILE and therefore try to un unlock() an AioContext lock
that was never acquired.

RHBZ: https://bugzilla.redhat.com/show_bug.cgi?id=2167838

Steps to reproduce the issue: simply boot a VM with
-object '{"qom-type":"iothread","id":"iothread1"}' \
-blockdev '{"driver":"file","filename":"$QCOW2","aio":"native","node-name":"libvirt-1-storage","cache":{"direct":true,"no-flush":false},"auto-read-only":true,"discard":"unmap"}' \
-blockdev '{"node-name":"libvirt-1-format","read-only":false,"cache":{"direct":true,"no-flush":false},"driver":"qcow2","file":"libvirt-1-storage"}' \
-device virtio-blk-pci,iothread=iothread1,drive=libvirt-1-format,id=virtio-disk0,bootindex=1,write-cache=on

and observe that it will fail not manage to boot with "qemu_mutex_unlock_impl: Operation not permitted"

Signed-off-by: Emanuele Giuseppe Esposito <eesposit@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Tested-by: Lukáš Doktor <ldoktor@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20230208111148.1040083-1-eesposit@redhat.com>