x86/HVM: use fixed TSC value when saving or restoring domain

x86/HVM: use fixed TSC value when saving or restoring domain

When a domain is saved each VCPU's TSC value needs to be preserved. To get it we
use hvm_get_guest_tsc(). This routine (either itself or via get_s_time() which
it may call) calculates VCPU's TSC based on current host's TSC value (by doing a
rdtscll()). Since this is performed for each VCPU separately we end up with
un-synchronized TSCs.

Similarly, during a restore each VCPU is assigned its TSC based on host's current
tick, causing virtual TSCs to diverge further.

With this, we can easily get into situation where a guest may see time going
backwards.

Instead of reading new TSC value for each VCPU when saving/restoring it we should
use the same value across all VCPUs.

Reported-by: Philippe Coquard <philippe.coquard@mpsa.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>