qom: Don't pass string table to object_get_enum() function

Now that properties can be explicitly registered as an enum
type, there is no need to pass the string table to the
object_get_enum() function. The object property registration
already has a pointer to the string table.

In changing this method signature, the hostmem backend object
has to be converted to use the new enum property registration
code, which simplifies it somewhat.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>