xen-platform: convert to memory API

Since this device bypasses PCI and registers I/O ports directly with
the system bus, it needs further attention.

Reviewed-by: Richard Henderson <rth@twiddle.net>
Reviewed-by: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
