libvirt: convert test_dmcrypt to avoid DB usage

Nothing in the dmcrypt test case uses the DB, so it was
incurring the DB setup penalty for no gain. Switch to
use NoDBTestCase reducing time from 1.7s to 0.09s

Partial-bug: #1369516
Change-Id: Ifd4b919f60f48f691d83cedb6ce7ea18c98636ad
Merge "libvirt: convert test_dmcrypt to avoid DB usage"