Support for libvirt/qemu tar releases

Enable the possibility to install libvirt and qemu from tar
releases. Defining in local.conf the variables LIBVIRT_LINK,
LIBVIRT_PY_LINK and QEMU_LINK will result in the compilation
and of the program sources from the tar files.

Example:
QEMU_LINK=http://wiki.qemu-project.org/download/qemu-2.1.0-rc3.tar.bz2
LIBVIRT_LINK=http://libvirt.org/sources/libvirt-1.2.6.tar.gz
LIBVIRT_PY_LINK=http://libvirt.org/sources/python/libvirt-python-1.2.6.tar.gz

Change-Id: I89d20609fbfc8791892ab103c7b25c1334e3e94d
Signed-off-by: Michele Paolino <m.paolino@virtualopensystems.com>