libxl: Allow libxl to find pygrub binary.

libxl: Allow libxl to find pygrub binary.

Specifying an explicit path to pygrub (e.g. BINDIR "/pygrub") only works if
Xen and libvirt happen to be installed to the same prefix. A more flexible
approach is to simply specify "pygrub" which will cause libxl to use the
correct path which it knows (since it is built with the same prefix as pygrub).

This is particular problematic in the Debian packaging, since the Debian Xen
package relocates pygrub into a libexec dir, however I think this change makes
sense upstream.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: Jim Fehlig <jfehlig@suse.com>