rpm: create libvirt-wireshark sub-package

rpm: create libvirt-wireshark sub-package

On Fedora 20, with wireshark-devel installed, 'make rpm' failed
due to installed but unpackaged files related to wireshark.  As
F20 is already released without wireshark, I chose to add a new
sub-package that is enabled only for F21 and later.  Furthermore,
all existing wireshark plugins belong to the wireshark package,
so I got to invent behavior of how the first third-party wireshark
module will behave.

* libvirt.spec.in (with_wireshark): Add new conditional.
* configure.ac (ws-plugindir): Improve wording.

Signed-off-by: Eric Blake <eblake@redhat.com>