commit 09778e09 switched from using ioctl(SIOCBRDELBR) for bridge
device deletion to using a netlink RTM_DELLINK message, which is the
more modern way to delete a bridge (and also doesn't require the
bridge to be ~IFF_UP to succeed). However, although older kernels
(e.g. 2.6.32, in RHEL6/CentOS6) support deleting *some* link types
with RTM_NEWLINK, they don't support deleting bridges, and there is no
compile-time way to figure this out.

This patch moves the body of the SIOCBRDELBR version of
virNetDevBridgeDelete() into a static function, calls the new function
from the original, and also calls the new function from the
RTM_DELLINK version if the RTM_DELLINK message generates an EOPNOTSUPP
error. Since RTM_DELLINK is done from the subordinate function
virNetlinkDelLink, which is also called for other purposes (deleting a
macvtap interface), a function pointer called "fallback" has been
added to the arglist of virNetlinkDelLink() - if that arg != NULL, the
provided function will be called when (and only when) RTM_DELLINK
fails with EOPNOTSUPP.

Resolves:  https://bugzilla.redhat.com/show_bug.cgi?id=1252780 (part 2)