Move more hypervisor strings into constants

This patch adds constants for the mouse and boot order strings; while
there are still some issues remaining, we're trying to cleanup hardcoded
strings from the hypervisors.

Since the formatting of frozensets is currently wrong, we also add an
utility function for this and change all the error messages to use it.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>