Revert "bhyve: add command line parameter and parsing for migration"

Unfortunately, this feature didn't receive much feedback in the past.
However, after committing this, some people came up and complain that
this feature requires some more discussion before upstreaming it.
Additionally, it wasn't a good idea to start this new feature by adding
a new command line parameter as it fixes the user interface.

This reverts commit c9fdd4f3cc18c03683de85318ba8d318f96b58c4.