VMX: don't allow PVH to reach handle_mmio()

PVH guests accessing I/O ports via string ops is not supported yet.

Reported-by: Roger Pau Monné <roger.pau@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Mukesh Rathor <mukesh.rathor@oracle.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Kevin Tian <kevin.tian@intel.com>
Release-Acked-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>