Remove duplicated tapdev creation code from libvirt VIF

Move the 'create_tap_dev' method out of the linux_net class
QuantumLinuxBridgeInterfaceDriver, to be a standalone method.
Make the libvirt LibvirtOpenVswitchDriver call this method
instead of duplicating the tapdev creation code

Blueprint: libvirt-vif-driver
Change-Id: Ib9b5fb3aed5e339464c4f367179911166f2cddfb
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Merge "Remove duplicated tapdev creation code from libvirt VIF"