virsh: domain: Don't use vshPrepareDiskXML for creating XML to detach disk

virsh: domain: Don't use vshPrepareDiskXML for creating XML to detach disk

Since cmdDetachDisk() calls into vshPrepareDiskXML() with
type == VSH_PREPARE_DISK_XML_NONE && source == NULL this would result
into skipping all the checks and effectively turn the function into a
XML formatter.

This patch changes the code to use the formatter directly so that the
function can be refactored in a easier way.