bhyve: add nvlist functions for setting unset nodes

If an emulation uses those functions instead of set_config_value_node
or set_config_value, it allows the config values to get
overwritten. Introducing new functions is much more readable than
if else statements in the emulation code.

Reviewed by: khng
MFC after: 2 weeks
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D33770