x86/pv: Make the PV default WRMSR path match the HVM default

x86/pv: Make the PV default WRMSR path match the HVM default

The current HVM default for writes to unknown MSRs is to inject #GP if the MSR
is unreadable, and discard writes otherwise. While this behaviour isn't great,
the PV default is even worse, because it swallows writes even to non-readable
MSRs.  i.e. A PV guest doesn't even get a #GP fault for a write to a totally
bogus index.

Update PV to make it consistent with HVM, which will simplify the task of
making other improvements to the default MSR behaviour.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>