pc-dimm: factor out capacity and slot checks into MemoryDevice

pc-dimm: factor out capacity and slot checks into MemoryDevice

Move the checks into memory_device_get_free_addr(). This will check
before doing any calculations if we have KVM/vhost slots left and if
the total region size would be exceeded.

Of course, while at it, make it independent of pc-dimm code.

Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20180423165126.15441-7-david@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>