Do not leak resources during attach if nvme_ctrlr_construct() or the initial
controller resets fail.

Sponsored by:	Intel
Reviewed by:	carl
Approved by:	re (hrs)
MFC after:	1 week
Change the behavior of bhyve such that the gdb listening port is opt-in
rather than opt-out.

Prior to this change if the "-g" option was not specified then a listening
socket for tunneling gdb packets would be opened at port 6466. If a second
virtual machine is fired up, also without the "-g" option, then that would
fail because there is already a listener on port 6466.

After this change if a gdb tunnel port needs to be created it needs to be
explicitly specified with a "-g <portnum>" command line option.

Reviewed by:	grehan@
Approved by:	re@ (blanket)