the `moby` tool uses a prefix for the build output. Specify the prefix
(and optional path) in the `Moby` property in the JSON instead of assuming
a different path for each initrd/kernel.
Also, adopt the new name for the kernel image (vmlinuz64 -> <prefix>-bzImage).
This should make it simpler to boot up hyperkit with Moby images
build from by the moby tool.
Signed-off-by: Rolf Neugebauer <rolf.neugebauer@docker.com>