memory: Provide separate handling of unassigned io ports accesses

memory: Provide separate handling of unassigned io ports accesses

Accesses to unassigned io ports shall return -1 on read and be ignored
on write. Ensure these properties via dedicated ops, decoupling us from
the memory core's handling of unassigned accesses.

Cc: qemu-stable@nongnu.org
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
(cherry picked from commit 3bb28b7208b349e7a1b326e3c6ef9efac1d462bf)

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>