Don't drop model=virtio when specifying bridge
Don't drop model=virtio when specifying bridge

When a user passes --bridge, the code strips the interface
element to replace the bridge name, but also drops the
model=virtio type.   Fix that by re-organizing the
bridge device.append to construct the model attribute.

Signed-off-by: Ryan Harper <7c8ea25f69f5166c51151e84bf5db678c2384828@canonical.com>

basak/uvtool,basak/uvtool