The current implementation of vmtest has a few issues:
1. Building drgn for each kernel version on Travis is slow, mostly
because they don't all run in parallel.
2. For local, incremental testing, recreating the filesystem image and
rebuilding drgn is slow, and syncing the code to the filesystem image
is brittle.
3. The filesystem image is the only communication channel, and reading
the exit status from the filesystem image is awkward and fragile.
4. Creating and accessing the filesystem image requires root.
This reworks vmtest to use the build on the host via VirtFS with a
simple agent on the guest that can execute arbitrary commands and return
the exit status. This has a few more moving parts but is faster and
saner overall.
