Make LibvirtDriver.uri() a staticmethod.

LibvirtDriver's uri() function seems to be the most logical place
where one can access the libvirt URL used by Nova. I would like
access to this outside the scope of the internal libvirt
driver (for libguestfs) so exposing it as a static
resource seems to make sense.

Partial fix for LP Bug #1131768.

Change-Id: I653296414716b4e799d68f6bfc868e6039d6ea4a
Merge "Make LibvirtDriver.uri() a staticmethod."