qemu.tests: Bug fix about drift for timedrift_with_stop

kvm guests CLOCK_MONOTONIC not count when guest is paused,
so drift need to subtract stop_time.

starting from Win2012 Windows no longer relies on number of
injected RTC interrupts for time keeping, and the time drift
is seen because "stop" monitor command also stops PM timer,
so this is unsupported scenario from Win2012 and disabled this
case from Win2012.

changes from v1:
    add some comments for windows guest.
changes from v2:
    add check rtc_clock for windows
changes from v3:
    add new scenario rtc_clock = host and rtc_clock = vm for this case.

Signed-off-by: Shuping Cui <scui@redhat.com>
qemu.tests: bug fix for timedrift_with_cpu_offline

add stop_time after set cpu offline, and update "smp_min" to "smp"
in config file.

changes from v1:
    drift_threshold type fix

Signed-off-by: Shuping Cui <scui@redhat.com>