tools/kvm_stat: rename 'Current' column to 'CurAvg/s'

'Current' can be misleading as it doesn't tell whether this is the amount
of events in the last interval or the current average per second.
Note that this necessitates widening the respective column by one more
character.

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