brSetInetAddress can only set a single IP address on the bridge, and
uses a method (ioctl(SIOCSETIFADDR)) that only works for IPv4. Replace
it and brSetInetNetmask with a single function that uses the external
"ip addr add" command to add an address/prefix to the interface - this
supports IPv6, and allows adding multiple addresses to the interface.

Although it isn't currently used in the code, we also add a
brDelInetAddress for completeness' sake.

Also, while we're modifying bridge.c, we change brSetForwardDelay and
brSetEnableSTP to use the new virCommand API rather than the
deprecated virRun, and also log an error message in bridge_driver.c if
either of those fail (previously the failure would be completely
silent).