amis: switch back to sda for root PV volume, add publish wrapper

If additional EBS volumes are mapped to a PV instance using a "sd*" name
they will always be ordered by the hypervisor before "xvd*" devices,
again ignoring the root device definition. This applies to all PV
instance types so we cannot get away with just poo-pooing m1.small.

We will need to call attention to this since it requires users who set
the volume size via APIs to use the name "/dev/sda" again.
Merge pull request #311 from marineam/ami

amis: switch back to sda for root PV volume, add publish wrapper