Let's change URI to parallels:///system. Parallels Server supports
creating VMs from non-privileged accounts, but it's not main usage
scenario and it may be forbidden in the future.

Also containers, which will be supported by the driver, can be managed
only by root, so /system path is more suitable for this driver.
Signed-off-by: NDmitry Guryanov <dguryanov@parallels.com>