awsutils: use /dev/xvd[a-z][a-z] for ephemeral drives

Better fix for gh-213. Use /dev/xvd[a-z][a-z] for ephemeral drives in the
runtime mapping in EasyEC2.request_instances. This avoids collision
with potential EBS volume mappings (ie /dev/sd[a-z]). This also avoids
needing to special-case the ephemeral drive naming scheme for
hs1.8xlarge because /dev/xvd[a-z][a-z] doesn't cause an
Internal.ServerError when launching hs1.8xlarge instances and seems to
work for all other instance types (tested on t1.micro, m1.small,
m1.xlarge, hs1.8xlarge).

Removed device_fmt kwarg from EasyEC2.create_block_device_map. This
function now handles the ephemeral device naming scheme internally.