pc: Factor out (un)plug handling of virtio-md-pci devices

Let's factor out (un)plug handling, to be reused from arm/virt code.

Provide stubs for the case that CONFIG_VIRTIO_MD is not selected because
neither virtio-mem nor virtio-pmem is enabled. While this cannot
currently happen for x86, it will be possible for arm/virt.

Message-ID: <20230711153445.514112-3-david@redhat.com>
Tested-by: Mario Casquero <mcasquer@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
diff --git a/MAINTAINERS b/MAINTAINERS
index bb4626f..12e59b6 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -2234,6 +2234,7 @@
 S: Supported
 F: hw/virtio/virtio-md-pci.c
 F: include/hw/virtio/virtio-md-pci.h
+F: stubs/virtio-md-pci.c
 
 virtio-mem
 M: David Hildenbrand <david@redhat.com>