virtio: Protect vqs list access

[ Upstream commit 0e566c8f ]

VQs may be accessed to mark the device broken while they are
created/destroyed. Hence protect the access to the vqs list.

Fixes: e2dcdfe9

 ("virtio: virtio_break_device() to mark all virtqueues broken.")
Signed-off-by: Parav Pandit <parav@nvidia.com>
Link: https://lore.kernel.org/r/20210721142648.1525924-4-parav@nvidia.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>