[Libguestfs] [REPOST] Use grub entries to find Linux kernels

This is a repost of the patch I sent on Friday. I know it was already ACK'd, but I've changed a fair bit in addition to adding the requested POD. Specifically: I discovered that augeas's grub lens wasn't returning cmdline the way I expected. This is fixed. The kernel package, if known, is output as a property of the kernel. The XML output of virt-inspector is updated with all the new output.