This refactors the TAP creation code out of brAddTap into a new
function brCreateTap to allow it to be used on its own. I have also
changed ifSetInterfaceMac to brSetInterfaceMac and exported it since
it is will be needed by code outside of util/bridge.c in the next
patch.

 AUTHORS                 |    1 +
 src/libvirt_bridge.syms |    2 +
 src/util/bridge.c       |  116 +++++++++++++++++++++++++++++++----------------
 src/util/bridge.h       |    9 ++++
 4 files changed, 89 insertions(+), 39 deletions(-)