Move _is_mapping to more central location

The logic to identify volumes is currently a nested function
in _default_block_device_names, named _is_mapping. It should
be moved to a more general location so others could utilize it
and allow it to be properly unit tested. The following patch
moves _is_mapping to nova/virt/block_device.py and renames it
to is_block_device_mapping.

Change-Id: I560abc4b57ca5bd195282af7cd1ab9bbf7600b67
Closes-Bug: #1351810
Merge "Move _is_mapping to more central location"