KVM: kvmtrace: replace get_cycles with ktime_get v3

The current kvmtrace code uses get_cycles() while the interpretation would be
easier using using nanoseconds. ktime_get() should give at least the same
accuracy as get_cycles on all architectures (even better on 32bit archs) but
at a better unit (e.g. comparable between hosts with different frequencies.

[avi: avoid ktime_t in public header]

Signed-off-by: Christian Ehrhardt <ehrhardt@linux.vnet.ibm.com>
Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Avi Kivity <avi@qumranet.com>