libxl: add explicit linkage to xenstore library

libxl: add explicit linkage to xenstore library

Since

  commit eee7bd4ecb5a3e83ed47cc89bf0dc4866b4858af
  Author: Joao Martins <joao.m.martins@oracle.com>
  Date:   Tue Jul 26 00:45:14 2016 +0100

    libxl: implement virDomainBlockStats

    Introduce initial support for domainBlockStats API

the libxl driver calls a couple of xenstore APIs, so it must explicitly
link to this library rather than rely on indirect linkage via libxl or
other xen libraries.

Reviewed-by: Jim Fehlig <jfehlig@suse.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>