bhyve: Some fwctl simplifications.

• Collapse IDENT_SEND/IDENT_WAIT states down to a single state.

• Remove unused 'len' argument to op_data callback. The value passed in (total amount of remaining data to receive) didn't seem very useful and no op_data implementations used it.

Reviewed by: corvink, markj
Differential Revision: https://reviews.freebsd.org/D41286