[libvirt] Add a ./run script for running programs from the local directory.

In libguestfs we successfully use a ./run script which sets up the environment so you can run libguestfs programs (internal and external ones, in C and non-C) without needing to install anything. This adds a similar script for libvirt. Here is the documentation included in the script: # With this script you can run libvirt programs without needing to # install them first. You just have to do for example: # # ./run ./tools/virsh [args ...] # # If you are already in the tools/ subdirectory, then the following # command will also work: # # ../run ./virsh [...] # # You can also run the C programs under valgrind like this: # # ./run valgrind [valgrind opts...] ./program # # or under gdb: # # ./run gdb --args ./program # # This also works with sudo (eg. if you need root access for libvirt): # # sudo ./run ./tools/virsh list --all