[Libguestfs] [PATCH supermin] Add support for a DAX root filesystem.

DAX is explained in detail here: https://lwn.net/Articles/610174/ This patch adds support to supermin for using a DAX root filesystem. The corresponding libguestfs patches will be posted shortly once I've tested them a bit more. This requires qemu >= 2.6. Unfortunately it's not really a win for a few reasons: - Requires enabling ACPI, which slows everything down by hundreds of milliseconds. I need to work out exactly what's going on here. Since we never required ACPI before now, I never had the impetus to find out if this is easy to fix. - The extra modules in the supermin mini-initrd greatly increase its size, which also slows things down. qemu fw_cfg DMA should help here, if we ever manage to get it upstream. - vNVDIMM and DAX are not very widely tested or used yet. - Not supported at all by libvirt. Rich.