Code review comment for lp://staging/~al-maisan/launchpad/bq-platform-columns-505725

Hello there!

In order to be able to formulate reasonably straightforward queries that find
candidate jobs for idle builders we'd like to add NULLable 'processor' and
'virtualized' columns to BuildQueue.

In case of binary builds these will replicate the respective Build.processor
and Build.is_virtualized values. For processor-independent jobs they will
remain NULL.

Please note also that these columns will only be initialed once, when the
BuildQueue record is inserted.

Pre-implementation chat with jml. Bug #505725.