xen/time: fix PV clock resolution

The current resolution of the Xen PV clock is too high, which causes an
adjustment of 5s to be applied to it. Reduce the resolution to be the same
as the RTC plus one, so it's always selected as the best source when
available on x86.

Also don't reset the clock on resume, it's pointless and discards any
previous adjustments.

Sponsoted by: Citrix Systems R&D