pvh: vmx-specific changes

pvh: vmx-specific changes

Changes:
* Enforce HAP mode for now
* Disable exits related to virtual interrupts or emulated APICs
* Disable changing paging mode
 - "unrestricted guest" (i.e., real mode for EPT) disabled
 - write guest EFER disabled
* Start in 64-bit mode
* Paging mode update to happen in arch_set_info_guest

Signed-off-by: Mukesh Rathor <mukesh.rathor@oracle.com>
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>