x86/pv: handle writes to the EFER MSR

x86/pv: handle writes to the EFER MSR

Silently drop writes to the EFER MSR for PV guests if the value is not
changed from what it's being reported. Current PV Linux will attempt
to write to the MSR with the same value that's been read, and raising
a fault will result in a guest crash.

As part of this work introduce a helper to easily get the EFER value
reported to guests.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>