qemu: command: Report stderr from qemu-bridge-helper
    
    There's a couple reports of things failing in this area (bug 1259070),
    but it's tough to tell what's going wrong without stderr from
    qemu-bridge-helper. So let's report stderr in the error message
    
    Couple new examples:
    
    virbr0 is inactive:
    internal error: /usr/libexec/qemu-bridge-helper --use-vnet --br=virbr0 --fd=21: failed to communicate with bridge helper: Transport endpoint is not connected
    stderr=failed to get mtu of bridge `virbr0': No such device
    
    bridge isn't on the ACL:
    internal error: /usr/libexec/qemu-bridge-helper --use-vnet --br=br0 --fd=21: failed to communicate with bridge helper: Transport endpoint is not connected
    stderr=access denied by acl file