vmd(8): don't remove known vm's from the config on error.

vmd(8): don't remove known vm's from the config on error.

Multiple error paths, specifically the one related to if a guest
cannot allocate memory at start, resulted in a known vm (via
vm.conf(5)) being removed from the vm list. Adjust the error paths
to check if the failing vm is defined in the config before tearing
it down.

Tested with help from beck@ and Mischa Peters.

ok beck@