x86/viridian: Add partition time reference counter MSR support

x86/viridian: Add partition time reference counter MSR support

This patch optionally re-instates support for the partition time reference
counter that was previously introduced by commit
e36cd2cdc9674a7a4855d21fb7b3e6e17c4bb33b and reverted by commit
1cd4fab14ce25859efa4a2af13475e6650a5506c. The previous implementation was
non-optional and flawed.

This implementation uses the tsc of vcpu0, which is preserved across
save/restore as part of the architectural state, and then converts that
to a 100ns tick using the domain's tsc_khz.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Cc: Keir Fraser <keir@xen.org>
Acked-by: Jan Beulich <jbeulich@suse.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Christoph Egger <chegger@amazon.de>
Acked-by: Ian Campbell <ian.campbell@citrix.com>