xen/io: provide helpers for multi size MMIO accesses

xen/io: provide helpers for multi size MMIO accesses

Several handlers have the same necessity of reading or writing from or to
an MMIO region using 1, 2, 4 or 8 bytes accesses.  So far this has been
open-coded in the function itself.  Instead provide a new set of handlers
that encapsulate the accesses.

Since the added helpers are not architecture specific, introduce a new
generic io.h header.

No functional change intended.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>