xl: allow check-xl-disk-parse to run against installed xl as well as build dir

xl: allow check-xl-disk-parse to run against installed xl as well as build dir

I can't run from the current directory since my build box isn't running Xen so
if ./xl doesn't exist use the installed version on the assumption that I've
copied the script to a test host.

I think running from the build dir needs the blktap2 libraries, so update
LD_LIBRARY_PATH as appropriate.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Ian Jackson <ian.jackson.citrix.com>
Committed-by: Ian Jackson <ian.jackson.citrix.com>