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 <paul.durrant@citrix.com>
Cc: Stefano Stabellini <sstabellini@kernel.org>
Cc: Anthony Perard <anthony.perard@citrix.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <1462811480-16295-1-git-send-email-paul.durrant@citrix.com>
Acked-by: Anthony PERARD <anthony.perard@citrix.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>