Support for custom git repositories for qemu and libvirt

Added basic support for custom libvirt and qemu repositories.
Defining in local.conf the variables:
- LIBVIRT_REPO and LIBVIRT_BRANCH for libvirt
- LIBVIRT_PY_REPO and LIBVIRT_PY_BRANCH for libvirt-python[1]
- QEMU_REPO and QEMU_BRANCH for qemu

will result in the compilation and of the program sources from git.

[1] This is mandatory when LIBVIRT_REPO is set, because in this
case the installation of the libvirt-python's dependencies from
the OS packages manager may conflict with the compiled libvirt.

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