Since kvm-kmod-2.6.32.2 we have an alternative source for recent KVM
kernel headers. Use it when available and not overruled by --kerneldir.
Signed-off-by: NJan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>