libxenstat: honour XEN_RUN_DIR

libxenstat: honour XEN_RUN_DIR

This is because libxl uses XEN_RUN_DIR to generate the socket path for
libxenstat while libxenstat itself uses hard-coded path, which is not
necessarily the same path as XEN_RUN_DIR.  The default configuration
happened to work because XEN_RUN_DIR defaulted to /var/run/xen, which
matched the hard-coded path.

We should make libxenstat use XEN_RUN_DIR so that it works with
non-default configuration.

Generate a _paths.h because it is required to make this change work.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
(cherry picked from commit dedb221889dbdd96f1d3c1155c3eb492d329bb53)