dump: Rework filter area variables

While the DumpState begin and length variables directly mirror the API
variable names they are not very descriptive. So let's add a
"filter_area_" prefix and make has_filter a function checking length > 0.

Signed-off-by: Janosch Frank <frankja@linux.ibm.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20220811121111.9878-6-frankja@linux.ibm.com>