Fix -a opt to boot-seed-vm.

Fixes an issue in 444c63128d0354c97d66a3bb5b8e23ea39633bbb where
we changed the behaviour of the -a argument. The -a option requires
an argument as well...

Change-Id: I68f8d5b0ea5d13b5136ae854e681892d48eeda90
Merge "Fix -a opt to boot-seed-vm."