tools/kvm_stat: separate drilldown and fields filtering

Drilldown (i.e. toggle display of child trace events) was implemented by
overriding the fields filter. This resulted in inconsistencies: E.g. when
drilldown was not active, adding a filter that also matches child trace
events would not only filter fields according to the filter, but also add
in the child trace events matching the filter. E.g. on x86, setting
'kvm_userspace_exit' as the fields filter after startup would result in
display of kvm_userspace_exit(DCR), although that wasn't previously
present - not exactly what one would expect from a filter.
This patch addresses the issue by keeping drilldown and fields filter
separate. While at it, we also fix a PEP8 issue by adding a blank line
at one place (since we're in the area...).
We implement this by adding a framework that also allows to define a
taxonomy among the debugfs events to identify child trace events. I.e.
drilldown using 'x' can now also work with debugfs. A respective parent-
child relationship is only known for S390 at the moment, but could be
added adjusting other platforms' ARCH.dbg_is_child() methods
accordingly.
Signed-off-by: Stefan Raspl <raspl@linux.vnet.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>