vmd(8): fix console attach from vmctl(8).

vmd(8): fix console attach from vmctl(8).

Adding in the ability to override the boot kernel created an edge
case in the ipc message handling logic for the parent process (vmd)
when receiving a "start vm" request. Result was incorrectly responding
to the control process, and as a result the vmctl client, with a
bogus "start vm response" reply with an empty tty name.

This commit rewrites the logic of how vmd goes about processing the
"start vm" request with the aim of making it simpler to understand
while addressing the edge case.

Issue reported by kn@. OK mlarkin@.