tools/kvm_stat: Exempt time-based counters

The new counters halt_poll_success_ns and halt_poll_fail_ns do not count
events. Instead they provide a time, and mess up our statistics. Therefore,
we should exclude them.
Removal is currently implemented with an exempt list. If more counters like
these appear, we can think about a more general rule like excluding all
fields name "*_ns", in case that's a standing convention.

Signed-off-by: Stefan Raspl <raspl@linux.ibm.com>
Tested-and-reported-by: Christian Borntraeger <borntraeger@de.ibm.com>
Message-Id: <20201208210829.101324-1-raspl@linux.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>