x86/hvm: Tidy up the viridian code a little and flesh out the APIC

x86/hvm: Tidy up the viridian code a little and flesh out the APIC
assist MSR handling code.

We don't say we that handle that MSR but Windows assumes it. In
Windows 7 it just wrote to the MSR and we used to handle that
ok. Windows 8 also reads from the MSR so we need to keep a record of
the contents.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>