block: protect BlockBackend->queued_requests with a lock

block: protect BlockBackend->queued_requests with a lock

The CoQueue API offers thread-safety via the lock argument that
qemu_co_queue_wait() and qemu_co_enter_next() take. BlockBackend
currently does not make use of the lock argument. This means that
multiple threads submitting I/O requests can corrupt the CoQueue's
QSIMPLEQ.

Add a QemuMutex and pass it to CoQueue APIs so that the queue is
protected. While we're at it, also assert that the queue is empty when
the BlockBackend is deleted.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Hanna Czenczek <hreitz@redhat.com>
Message-Id: <20230307210427.269214-4-stefanha@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>