virtio_pci_modern: remove admin queue serialization lock

The admin queue operations are protected by newly introduced
spin lock. To make it possible to issue parallel commands, remove the
admin queue serialization lock.

Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Message-Id: <20240716113552.80599-14-jiri@resnulli.us>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>