pc-dimm: merge get_(vmstate_)memory_region()

pc-dimm: merge get_(vmstate_)memory_region()

Importantly, get_vmstate_memory_region() should also fail with a proper
error if called before the device is realized. For a PCDIMM, both functions
are to return the same thing, so share the implementation.

All current users are called after the device has been realized, so we
can expect the calls to succeed.

Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20180619134141.29478-9-david@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>