[libvirt] libvirt needs to dynamically support kvm-img and/or qemu-img

Several distros out there install "kvm-img" when they install KVM and do not install "qemu-img". The following patch checks for kvm-img and qemu-img and does the right thing. I've taken it a step further and made the check a run-time check instead of a build-time check since people are able to remove and install KVM, QEMU and Xen components as they please and as such may invalidate the build-time check. The patch prefers "kvm-img" over "qemu-img" over "qcow-create" since there was a comment that libvirt wanted to support a feature of qemu-img that wasn't present in upstream qemu releases, however the feature is present in kvm-img. The patch adds a new util function called "virFindFileInPath()" which merely searches PATH for the actual location of the binary. It might be worth-while to make some of the other build-time tool checks run-time with that function since it would allow users to get more features just by installing new tools and wouldn't require distros to disable features because they don't want libvirt to be too dependency heavy. -- Doug Goldstein