tools/kvm_stat: add new interactive command 'b'

Toggle display total number of events by guest (debugfs only).
When switching to display of events by guest, field filters remain
active. I.e. the number of events per guest reported considers only
events matching the filters. Likewise with pid/guest filtering.
Note that when switching to display of events by guest, DebugfsProvider
remains to collect data for events as it did before, but the read()
method summarizes the values by pid.

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