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.
Use sd[f-z] for raid devices instead of sdf[1..].  AWS HVM images do not support sdf1 styledevice naming.
ebs_raid hvm support bug fixes
Merge remote-tracking branch 'origin/pr/63'
Merge remote-tracking branch 'origin/pr/73'