libvirt.tests: Fix the boundary test issue in nwfilter define cases

With libvirt commit 4f20943 in upstream, nwfilter check strict
boundary values when define filter. The RHEL did not include the
commit yet, this will cause different result as run on fedora and
RHEL. For fix this, add boundary error cases in negative cases,
and a new param 'boundary_test_skip' to enable skip the negative
boundary cases in RHEL or other unsupported strict check binaries.
The positive cases also updated with delete all boundary negative
values.

By default in test cfg, 'boundary_test_skip' is set as 'yes', which
will enable skip such negative boundary cases on RHEL, but fedora
will get pass as expected.

Updated use try..finally to make sure cleanup excuted finally.

The issue detail is recorded in:
https://github.com/autotest/tp-libvirt/pull/3/#issuecomment-34099397

Signed-off-by: Wayne Sun <gsun@redhat.com>