pvh: tolerate HVM guests having no ioreq page

pvh: tolerate HVM guests having no ioreq page

PVH guests don't have a backing device model emulator (qemu); just
tolerate this situation explicitly, rather than special-casing PVH.

For unhandled IO, hvmemul_do_io() will now return X86EMUL_OKAY, which
is I believe what would be the effect if qemu didn't have a handler
for the IO.

This also fixes a potetial DoS in the host from the reworked series:
If the guest makes a hypercall which sends an invalidate request, it
would have crashed the host.

Signed-off-by: George Dunlap <george.dunlap@eu.citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Keir Fraser <keir@xen.org>
Acked-by: Eddie Dong <eddie.dong@intel.com>