Build creation is restricted only to the PocketChroot presence, not the chroot itself

SPPH.createMissingBuilds() calculates distroarchseries availability by simply checking the presence of a corresponding PocketChroot record.

This is incompatible with the way chroots are handled, when they are deleted, the PocketChroot remains and only it's 'chroot' attribute it cleared.

IFW, createMissingBuilds() should use DAS.getChroot() instead of DAS.getPocketChroot().

This problem exposes a possible underlying issue with buildd-manager when it tries to dispatch a job for which it cannot find the chroot. It simply blocks, waiting an extra interaction with the targeted builder.