x86/pv: Hide more EFER bits from PV guests

x86/pv: Hide more EFER bits from PV guests

We don't advertise SVM in CPUID so a PV guest shouldn't be under the
impression that it can use SVM functionality, but despite this, it really
shouldn't see SVME set when reading EFER.

On Intel processors, 32bit PV guests don't see, and can't use SYSCALL.

Introduce EFER_KNOWN_MASK to whitelist the features Xen knows about, and use
this to clamp the guests view.

Take the opportunity to reuse the mask to simplify svm_vmcb_isvalid(), and
change "undefined" to "unknown" in the print message, as there is at least
EFER.TCE (Translation Cache Extension) defined but unknown to Xen.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Release-acked-by: Juergen Gross <jgross@suse.com>