Some of the block_device_mapping_v2 parameter descriptions
were lacking details on valid values, combinations, restrictions
and how things get linked together, for example, to get a swap
bdm. This fills in the details and re-words things in some
places for clarity.
The mention of the libvirt driver in the guest_format for
non-swap ephemeral block devices is removed since (1) it's
overly confusing and (2) API users shouldn't know or care
if the hypervisor running their server is libvirt or not.
The point is, if you want swap, ask for swap, and if you want
ephemeral, specify one of the other values. Then you get what
you get and you don't throw a fit.
Change-Id: Ia7e49f28e2f410ada5c2f4c2e3a13ffd97e54654
c8edb8ce72