qemuBuildUSBControllerDevStr: Format via JSON properties.

qemuBuildUSBControllerDevStr: Format via JSON properties.

Internally format the PCI controller properties into JSON, but convert
it back to a string as preparation for upcoming refactors.

The following types are declared for the properties we use by QEMU:

'nec-usb-xhci'
  p2=<uint32>            -  (default: 4)
  p3=<uint32>            -  (default: 4)

'ich9-usb-uhci6'
  masterbus=<str>
  firstport=<uint32>     -  (default: 0)

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>