test: add user_xattr check for securityselinuxlabeltest

test: add user_xattr check for securityselinuxlabeltest

libvirt unit test used setxattr with "user.libvirt.selinux" name to
emulate setfilecon of selinux. But for some old kernel filesystem
(like 2.6.32-431.el6.x86_64), if the filesystem is not mounted with
user_xattr flag, the setxattr with "user.libvirt.selinux" will fail.

So adding testUserXattrEnabled() in securityselinuxlabeltest.c,
if user_xattr is not enabled, skip this case.

The user_xattr is departed in newer kernel, therefore this commit is
only for the compatablity for old kernel.

Signed-off-by: Jincheng Miao <jmiao@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>
Tested-by: Scott Sullivan <ssullivan@liquidweb.com>