xen: Move tsc reliability check until after CPUs have booted

xen: Move tsc reliability check until after CPUs have booted

AMD CPUs by default enable X86_FEATURE_TSC_RELIABLE, and depend upon a
later check to disable this feature if TSC drift is detected.
Unfortunately, this check is done in time.c:init_xen_time(), which is
done before any secondary CPUs are brought up, and is thus guaranteed
to succed.

This patch moves the check into its own function, and calls it after
cpus are brought up.

Signed-off-by: George Dunlap <george.dunlap@eu.citrix.com>