memory-device: Track required and actually used memslots in DeviceMemoryState

memory-device: Track required and actually used memslots in DeviceMemoryState

Let's track how many memslots are required by plugged memory devices and
how many are currently actually getting used by plugged memory
devices.

"required - used" is the number of reserved memslots. For now, the number
of used and required memslots is always equal, and there are no
reservations. This is a preparation for memory devices that want to
dynamically consume memslots after initially specifying how many they
require -- where we'll end up with reserved memslots.

To track the number of used memslots, create a new address space for
our device memory and register a memory listener (add/remove) for that
address space.

Message-ID: <20230926185738.277351-9-david@redhat.com>
Reviewed-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>