Remove invalid block_device_mapping volume_size of ''

Fixes bug #1171190

Parsing block_device_mapping from strings sometimes results in
volume_size being set to '' or u''.  This causes a problem when
inserting into PostgreSQL, which insists that integer columns
actually contain integers.  Remove invalid device_size before
inserting or updating block_device_mapping database records.

Change-Id: If9132a96604b5b0e3a6df96624e565e016d5c9a3
Merge "Remove invalid block_device_mapping volume_size of ''"