Add a temporary (for values of "temporary") work around for hotplug
support with ath(4) and VIMAGE.

Right now the VIMAGE code doesn't supply a default vnet context during:

• hotplug attach;
• any device detach.

It special cases kldload/boot time probing (by setting the context to
vnet0) but that doesn't occur when probing devices during a bus rescan -
eg, adding a cardbus card.

These will eventually go away when the VIMAGE support extends to providing
default contexts to hotplug attach/detach.