qemu: Build usb controller command line more wisely
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1552127
    
    When building command line for USB controllers we have to do more
    than just put controller's alias onto the command line. QEMU has
    concept of these joined USB controllers. For instance ehci and
    uhci controllers need to create the same USB bus. To achieve that
    the slave controller needs to refer the master controller. This
    worked until we've introduced user aliases because both master
    and slave had the same alias. With user aliases slave can have
    different alias than master. Therefore, when generating command
    line for slave we need to look up the master's alias.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>