Advisory: XSA-193

Xen change c42494acb2 switched wrmsr_safe() for wr{f,g}sbase(), neglecting to consider that they internally may use plain wrmsr() or the wr{f,g}sbase instructions, both of which will suffer a #GP fault in this case for non-canonical addresses.

Check that Xen properly bounces the #GP faults back to us, rather than dying itself.

See also: tests/xsa-193/main.c