tools/kvm_stat: fix event counts display for interrupted intervals

When an update interval is interrupted via key press (e.g. space), the
'Current' column value is calculated using the full interval length
instead of the elapsed time, which leads to lower than actual numbers.
Furthermore, the value should be rounded, not truncated.
This is fixed by using the actual elapsed time for the calculation.

Signed-off-by: Stefan Raspl <raspl@linux.vnet.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>