mangled instance creation in virt drivers due to improper merge conflict resolution

[Matt Dietz]

Specifically the spawn() method in the virt drivers, as part of the migrations branch, was supposed to accept an optional argument containing disk metadata for spawning an instance with a pre-established disk. However, during one of the many merge conflict resolution sessions, the method was improperly reverted to the original signature.

After discussing offline, this not only needs to be fixed, but should be implemented that it doesn't impact the original spawn method.