bhyve: Use VMIO_SIOCSIFFLAGS instead of SIOCGIFFLAGS

Creating an IP socket to invoke the SIOCGIFFLAGS ioctl on is the only
thing preventing bhyve from working inside a bhyve jail with IPv4 and
IPv6 disabled restricting the jailed bhyve process to only access the
host network via a tap/vmnet device node.

PR: 273557
Fixes: 56be282bc999 ("bhyve: net_backends, automatically IFF_UP tap devices")
Reviewed by: markj
MFC after: 1 week