spec: Avoid RPM verification errors on nwfilter XMLs

spec: Avoid RPM verification errors on nwfilter XMLs

/etc/libvirt/nwfilter/*.xml files are installed with no UUID, which
means libvirtd will automatically alter all of them once it starts. Thus
RPM verification will always fail on them. Let's use a trick similar to
the default network XML and store nwfilter XMLs in /usr/share. They will
be copied into /etc in %post. Additionally the /etc files are marked as
%ghost so that they are uninstalled if the RPM package is removed.

Note that the %post script overwrites existing files with new ones on
upgrade, which is what has always been happening.

https://bugzilla.redhat.com/show_bug.cgi?id=1431581
https://bugzilla.redhat.com/show_bug.cgi?id=1378774

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>