[Libguestfs] [PATCH] Add 'make installcheck' rule to test installed packages.

This is my proposed alternative to the complicated test framework (https://www.redhat.com/archives/libguestfs/2015-August/msg00022.html). In this patch, when we want to run tests on installed packages we just copy in the installed binaries over the source directory, and run the test suite as normal. This is basically the same as the current (not used) contrib/make-check-on-installed.pl script, but with a bunch of fixes. Rich.