x86/HVM: XSETBV intercept needs to check CPL on SVM only

x86/HVM: XSETBV intercept needs to check CPL on SVM only

VMX doesn't need a software CPL check on the XSETBV intercept, and
SVM can do that check without resorting to hvm_get_segment_register().

Clean up what is left of hvm_handle_xsetbv(), namely make it return a
proper error code.

Signed-off-by: Jan Beulich <jbeulich@suse.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>