Fixes the somewhat embarrassing bug where the shutdown_command would be
run on the host rather than the guest.
This commit also rejigs things so that it is easier to do SSH commands
on either the host or the guest
Compiles -- but xva create_instance is incomplete
Several changes were rolled into this commit, including:
- "export_format" is now "format", and "keep_instance" is "keep_vm"
- gox is used for building (with the intention of allowing
cross-compilation in the future)
