pc-dimm: get_memory_region() will not fail after realize

pc-dimm: get_memory_region() will not fail after realize

Let's try to reduce error handling a bit. In the plug/unplug case, the
device was realized and therefore we can assume that getting access to
the memory region will not fail.

For get_vmstate_memory_region() this is already handled that way.
Document both cases.

Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20180619134141.29478-13-david@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>