php: Fix test, /dev/null format must be "raw".
php: Print libguestfs error string if test fails.
launch: libvirt: Print proper dotted libvirt version.
launch: libvirt: Unset $PATH can cause libvirt launch to fail.

Note this fact in the error message that is printed in this situation.
docs: Note that $PATH affects libguestfs.

If $PATH is not set, libguestfs and/or libvirt tend to break.
php: Revise README-PHP documentation.
php: Fix tests.

- Use ./run script to run the tests.
- Set environment variables correctly, including $PATH.
- Test the locally built, not installed, copy of libguestfs.