Fix docstring in libvirt.driver.LibvirtDriver.get_instance_disk_info()

Update param list to match reality and fold in pre-existing
NOTEs about the parameters into the doc string.

Change-Id: I994f75ef4d0f1540c007ec1d430c6973e9308e35
Merge "Fix docstring in libvirt.driver.LibvirtDriver.get_instance_disk_info()"