This patch implements three tests for kvmclock. First one check whether
the date of time returned by kvmclock matches the value got from
host. Second one check whether the cycle of kvmclock grows
monotonically in smp guest and the count of warps and stalls are also
recorded. The last test is a performance test to measure the guest
cycles cost when trying to read cycle of kvmclock.

Three parameters were accepted by the test: test loops, seconds
since 1970-01-01 00:00:00 UTC which could be easily get through date
+%s and the max accepted offset value between the tod of guest and
host.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>