Support XFS as ebs_raid filesystem. Tweak README.

This could be generalised to a single

    mkfs -t #{filesystem} #{md_device}

but this approach ensures xfs tools are available if required.
Restore comment about default ebs_raid FS.
mkfs doesn't need -F unlike mk2fs.
include xfs correctly for an LWRP.
Merge remote-tracking branch 'bazbremmer/add_xfs_raid'