Separate xennet(4) backend initialization code ("resume") from the part
that talks with Xenstore to query backend's information. Resuming is now
performed just after xennet(4) attachment instead of waiting for backend
to announce its features in Xenstore and change it state.

This fixes the race observed by Urban Boquist when the domU boots with
root on NFS.

FWIW, the boot code (when root is NFS-backed) can innit() the xennet(4)
interface very early: it tried to access ifnet structures that were not
yet allocated.

Will ask for a pullup. Thanks to Urban for reporting the issue and
investigate it. Confirmed fixed. No regression observed by me for
dynamic attach/detach of xvif(4) and xennet(4) interfaces.

See also http://mail-index.netbsd.org/port-xen/2011/04/18/msg006647.html

(jym)