byhve: add option to specify IP address for gdb

Allow user to specify the IP address available for gdb debugger.

Reviewed by: jhb, grehan, rgrimes, bcr (man pages)
Differential Revision: https://reviews.freebsd.org/D29607

(cherry picked from commit 2cdff9918e79dcd9bfcaa919b8364afa41cd1e02)