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

[Richard W.M. Jones]

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