introspective-instance-monitor:Should not use LIBVIRT_DEFAULT_URI in function checkGuests

[长孙弘奕]

in masakari-monitors/masakarimonitors/introspectiveinstancemonitor/qemu_utils.py:390
code ` conn = libvirt.open(None) # LIBVIRT_DEFAULT_URI`
will use the environment variable LIBVIRT_DEFAULT_URI.
If I run the masakari-introspective-instance-monitor service in ubuntu 18.04 with `systemctl restart masakari-introspective-instance-monitor`.LIBVIRT_DEFAULT_URI is qemu:///sessoin. and I can not get any domains.
However, in masakari-monitors/masakarimonitors/introspectiveinstancemonitor/instance.py:172,LOG debug tells to use uri with CONF.libvirt.connection_uri. It cost me a lot of time to troubleshooting the problem.
So, I think alter `conn = libvirt.open(None)` to `conn = libvirt.open(CONF.libvirt.connection_uri)` is better.

[leelswngsh (lswngsh)]

 the service introspectiveinstancemonitor continually output logs as below:

WARNING masakarimonitors.introspectiveinstancemonitor.qemu_utils [-] Failed to connect socket to '/var/lib/masakari/.cache/libvirt/libvirt-sock': No such file or directory: libvirt.libvirtError: Failed to connect socket to '/var/lib/masakari/.cache/libvirt/libvirt-sock': No such file or directory

WARNING masakarimonitors.introspectiveinstancemonitor.instance [-] Error from libvirt : Failed to connect socket to '/var/lib/masakari/.cache/libvirt/libvirt-sock': No such file or directory

[leelswngsh (lswngsh)]

in masakari-monitors/masakarimonitors/introspectiveinstancemonitor/qemu_utils.py:390
I think alter `conn = libvirt.open(None)` to `conn = libvirt.open(CONF.libvirt.connection_uri) and print the log with LOG.warning("lsw999 %s", print(type(running))).
Then ,I restart introspectiveinstancemonitor service,the log output indicates the "running" object is None;So the code in masakari-monitors/masakarimonitors/introspectiveinstancemonitor/qemu_utils.py:393

running = map(conn.lookupByID, ids) ；Is there a problem with this line of code？