isa: implement isa_is_ioport_assigned via memory_region_find

Open-code isa_is_ioport_assigned via a memory region lookup. As all IO
ports are now directly or indirectly registered via the memory API, this
becomes possible and will finally allow us to drop the ioport tables.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>