x86/HVM: introduce hvm_get_cpl() and respective hook

x86/HVM: introduce hvm_get_cpl() and respective hook

... instead of repeating the same code in various places (and getting
it  wrong in some of them).

In vmx_inst_check_privilege() also stop open coding
vmx_guest_x86_mode().

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Razvan Cojocaru <rcojocaru@bitdefender.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Acked-by: Kevin Tian <kevin.tian@intel.com>
Acked-by: Tim Deegan <tim@xen.org>