docker: add Xen libs to centos6 image

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20170728233316.13352-3-f4bug@amsat.org>
Signed-off-by: Fam Zheng <famz@redhat.com>

tests/docker/dockerfiles/centos6.docker

@@ -1,5 +1,5 @@
 FROM centos:6
-RUN yum install -y epel-release
+RUN yum install -y epel-release centos-release-xen
 ENV PACKAGES \
     bison \
     ccache \
@@ -13,6 +13,7 @@ ENV PACKAGES \
     pixman-devel \
     SDL-devel \
     tar \
+    xen-devel \
     zlib-devel
 RUN yum install -y $PACKAGES
 RUN rpm -q $PACKAGES | sort > /packages.txt