TSC_OFFSET will be adjusted if discovers TSC backward during vCPU load.
The preemption timer, which relies on the guest tsc to reprogram its
preemption timer value, is also reprogrammed if vCPU is scheded in to
a different pCPU. However, the current implementation reprogram preemption
timer before TSC_OFFSET is adjusted to the right value, resulting in the
preemption timer firing prematurely.

This patch fix it by adjusting TSC_OFFSET before reprogramming preemption
timer if TSC backward.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krċmář <rkrcmar@redhat.com>
Cc: Yunhong Jiang <yunhong.jiang@intel.com>
Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>