virtio: avoid using guest_notifier_mask in vhost-user mode

Because guest mask notifier cannot be used in vhost-user mode, a boolean
flag "use_guest_notifier_mask" was added in commit 5669655aafd to disable
the use of guest mask notifier under virtio-pci. However this flag wasn't
checked in other virtio devices, such as virtio-mmio. In our tests, it
caused assertion error under "vhost-user + virtio-mmio". This patch
addresses this problem by adding a check before guest_notifier_mask is
called.

Signed-off-by: Wei Huang <wei@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
