Controller: conditional VirtUtils import

Since running the lnst controller doesn't necessarily depend on having
libvirt running or installed this commit adds a condition to the import
of the VirtUtils module. This will fix lnst-ctl crashing on machines
that don't have libvirt running.

Signed-off-by: Ondrej Lichtner <olichtne@redhat.com>
Signed-off-by: Jiri Pirko <jiri@resnulli.us>
VirtUtils: move to lnst/Controller

VirtUtils are only used by the controller and there is no use case in
which we would want to use it on slaves. Therefore it makes more sense
to move it to the lnst/Controller directory.

Signed-off-by: Ondrej Lichtner <olichtne@redhat.com>
Signed-off-by: Jiri Pirko <jiri@resnulli.us>