tools/kvm_stat: simplify line print logic

Simplify line print logic for header and data lines in interactive mode
as previously suggested by Radim.
While at it, add a space between the first two columns to avoid the
total bleeding into the event name.
Furthermore, for column 'Current', differentiate between no events being
reported (empty 'Current' column) vs the case where events were reported
but the average was rounded down to zero ('0' in 'Current column), for
the folks who appreciate the difference.
Finally: Only skip events which were not reported at all yet, instead of
events that don't have a value in the current interval.
Considered using constants for the field widths in the format strings.
However, that would make things a bit more complicated, and considering
that there are only two places where output happens, I figured it isn't
worth the trouble.

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