Description:
While su - to usermode, the env parameter $XDG_RUNTIME_DIR is NULL，it should get the correct value automatically.

using  su -  and  ssh two different ways to switch to user mode  in two terminal,
guest status is different . the change to the guest can not be synchronized

Product Version:
kernel-3.10.0-188.el7.x86_64
qemu-kvm-rhev-2.1.2-3.el7.x86_64
libvirt-1.2.8-5.el7.x86_64

How producible:
100%

Steps:
1. root login into host ;

add a test user.
#useradd test

#pwd
/home/test

prepare a img and xml which belong to test .

# ls -al domain.xml r7new.img
-rw-r--r--. 1 test test       3281 Oct 16 11:09 domain.xml
-rw-r--r--. 1 test test 3863478272 Oct 16 11:07 r7new.img

2. in terminal 1, using ssh to switch to user mdoe:

#ssh -X test@localhost

$ virsh define domain.xml
Domain r7user defined from domain.xml


[test@184 ~]$ virsh start r7user
Domain r7user started

[test@184 ~]$ virsh list --all
 Id    Name                           State
----------------------------------------------------
 2     r7user                         running


[test@184 ~]$  echo $XDG_RUNTIME_DIR
/run/user/1001


3. in terminal2, using su - test to switch to user mode :

[root@184 test]# su - test

to check the guest status:

[test@184 ~]$ virsh list --all
 Id    Name                           State
----------------------------------------------------
 -     r7user                         shut off

check $XDG_RUNTIME_DIR, nothing out put.
[test@184 ~]$  echo $XDG_RUNTIME_DIR 


if start guest
#virsh start r7user
it will hang

ctrl + c to stop then

#virsh start r7user
error: Domain is already active

[test@184 ~]$ virsh list --all
 Id    Name                           State
----------------------------------------------------
 2     r7user                         shut off


4. check with qemu , it has two qemu process.
[test@184 ~]$ ps -ef | grep qemu
test      9567     1  1 11:56 ? .......
test      9601     1  0 11:56 ?.......


5. set the value of $XDG_RUNTIME_DIR manually same with the one in step 2, the guest status will be sync with step 2.
[test@184 ~]$ export XDG_RUNTIME_DIR='/run/user/1001'
[test@184 ~]$  echo $XDG_RUNTIME_DIR
/run/user/1001
[test@184 ~]$ virsh list --all
 Id    Name                           State
----------------------------------------------------
 2     r7user                         running


Actual Result:
In step 3, using  su -  switch to user mode , the environment parameter  XDG_RUNTIME_DIR is empty. libvirt guest can not get correct status .
  
Expected Result:
In step 3, using  su -  switch to user mode , the environment parameter  XDG_RUNTIME_DIR should get the cocrret value automatically. libvirt guest can get correct status. 
The guest status should be synced while using ssh and su to the usermode.


additonal information:
There is one similar bug for you reference, which is verified.
https://bugzilla.redhat.com/show_bug.cgi?id=955470