[ffx] Add Behavior processing to the qemu engine

Now that we have Behaviors in the manifest and the engine configuration,
we need to actually use them. This CL adds processing code to the Qemu
engine which iterates through the manifest Behaviors, runs filtering,
setup, argument parsing, and cleanup during shutdown.

Change-Id: Ibe5d77cbfd11c0e7b2f56194f17d1ca0ea20bddc
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/613639
Commit-Queue: Daniel Dougherty <doughertyda@google.com>
Reviewed-by: Clayton Wilkinson <wilkinsonclay@google.com>