x86/viridian: remove the viridian_vcpu msg_pending bit mask

x86/viridian: remove the viridian_vcpu msg_pending bit mask

The mask does not actually serve a useful purpose as we only use the SynIC
for timer messages. Dropping the mask means that the EOM MSR handler
essentially becomes a no-op. This means we can avoid setting 'message_pending'
for timer messages and hence avoid a VMEXIT for the EOM.

Signed-off-by: Paul Durrant <pdurrant@amazon.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Wei Liu <wl@xen.org>