tools/tests: add xenstore testing framework

tools/tests: add xenstore testing framework

Add tools/tests/xenstore for a framework to do tests of xenstore.
The aim is to test for correctness and performance.

Add a test program containing some tests meant to be run against any
xenstore implementation (xenstored, oxenstored, xenstore-stubdom).

It is using libxenstore for access to xenstore and supports multiple
tests to be either selected all or individually. All tests are using
/local/domain/<own-domid>/xenstore-test/<pid> as base for doing the
tests. This allows multiple instances of the program to run in
parallel.

Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>