x86/hvm: Improve error information in handle_pio()

x86/hvm: Improve error information in handle_pio()

domain_crash() should always have a message which is emitted even in release
builds, so something more useful than this is presented to the user.

  (XEN) domain_crash called from io.c:171
  (XEN) domain_crash called from io.c:171
  (XEN) domain_crash called from io.c:171
  ...

To avoid possibly printing stack rubble, initialise data to ~0 right away.
Furthermore, the maximum access size is 4, so drop data from long to int.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>