pvclock-abi: respect the TSC_STABLE_BIT flag

According to the ABI we should not assume the clock is monotonic
unless TSC_STABLE_BIT is set. When it's not, we use a global variable
to prevent clock backwards jumps.

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>