Added libvirt-dev[el] as a required system package

Added libvirt-dev as a system package to install with apt-get and
libvirt-devel as a system package to install with rpm.

Early drafts qualified with "testonly" (meaning to install only if
INSTALL_TESTONLY_PACKAGES is true).  This is needed because installing
libvirt-python version 1.2.5 on Ubuntu 14.04 was observed to fail in
the building step if libvirt-dev is missing.  Later drafts removed
that qualification, because Sean Dague said he thinks libvirt-dev[el]
is always required.

Change-Id: Ie6a272f60059a1f363630f307416b32c450a1ebb
Closes-Bug: 1362948
Merge "Added libvirt-dev[el] as a required system package"
Merge branch 'master' of https://github.com/openstack-dev/devstack