virtio-pmem: add virtio device

This is the implementation of virtio-pmem device. Support will require
machine changes for the architectures that will support it, so it will
not yet be compiled. It can be unlocked with VIRTIO_PMEM_SUPPORTED per
machine and disabled globally via VIRTIO_PMEM.

We cannot use the "addr" property as that is already used e.g. for
virtio-pci/pci devices. And we will have e.g. virtio-pmem-pci as a proxy.
So we have to choose a different one (unfortunately). "memaddr" it is.
That name should ideally be used by all other virtio-* based memory
devices in the future.
    -device virtio-pmem-pci,id=p0,bus=bux0,addr=0x01,memaddr=0x1000000...

Acked-by: Markus Armbruster <armbru@redhat.com>
[ QAPI bits ]
Signed-off-by: Pankaj Gupta <pagupta@redhat.com>
[ MemoryDevice/MemoryRegion changes, cleanups, addr property "memaddr",
  split up patches, unplug handler ]
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20190619094907.10131-2-pagupta@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>