x86/hvm: simplify hvmemul_do_io()

x86/hvm: simplify hvmemul_do_io()

Currently hvmemul_do_io() handles paging for I/O to/from a guest address
inline. This causes every exit point to have to execute:

if ( ram_page )
    put_page(ram_page);

This patch introduces wrapper hvmemul_do_io_addr() and
hvmemul_do_io_buffer() functions. The latter is used for I/O to/from a Xen
buffer and thus the complexity of paging can be restricted only to the
former, making the common hvmemul_do_io() function less convoluted.

This patch also tightens up some types and introduces pio/mmio wrappers
for the above functions with comments to document their semantics.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>