qemu: Build usb controller command line more wisely

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>