Uniformize some function names in backend.py

Currently, the names of the functions in backend.py that are actually
RPC procedures and are called from ganeti-noded are not corresponding to
the RPC names. This makes it hard to actually see which functions are
exported and which functions are internal to backend.

This patch renames all blockdevice-related functions in backend.py match
the name of the RPC call (without the ‘call’ or ‘perspective’ prefix).
This should make it easier to grep for a given function called in
cmdlib, without having to open and check in ganet-inoded what backend
function it corresponds to.

The patch also does two minor extra cleanups (rename a variable and
change a logging level).

Reviewed-by: ultrotter