xen-hvm: ignore background I/O sections

xen-hvm: ignore background I/O sections

Since Xen will correctly handle accesses to unimplemented I/O ports (by
returning all 1's for reads and ignoring writes) there is no need for
QEMU to register backgroud I/O sections.

This patch therefore adds checks to xen_io_add/del so that sections with
memory-region ops pointing at 'unassigned_io_ops' are ignored.

Signed-off-by: Paul Durrant <[email protected]>
Cc: Stefano Stabellini <[email protected]>
Cc: Anthony Perard <[email protected]>
Cc: Paolo Bonzini <[email protected]>
Message-Id: <1462811480[email protected]>
Acked-by: Anthony PERARD <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>