Revert "virttest.qemu_vm: Add freq option to usb device"

This reverts commit fadf99e6c0b1bdd86315040b3d3a35a804a550e8.

The freq option was removed in commit:
https://github.com/autotest/autotest/commit/93acc8819c8d912f2f3bf6f4d5d66bc2c99181a4
Thus revert this commit from virt-test.

Signed-off-by: Qingtang Zhou <qzhou@redhat.com>
virt.qemu_vm: Get usb device pci address with get_free_pci_addr()

Signed-off-by: Qingtang Zhou <qzhou@redhat.com>
virt.qemu_vm: Refactory get_free_usb_port function

This patch rewrites the get_free_usb_port function:
 * Moving it out of make_create_command function.
 * Creating some helper function.

Signed-off-by: Qingtang Zhou <qzhou@redhat.com>
virt.qemu_vm: Add support for usb-hub device

Signed-off-by: Qingtang Zhou <qzhou@redhat.com>
virt.qemu_vm: Add usb_bus/port parameters for usb device

With these 2 parameters you can assign a specific bus/port
for usb device.

Signed-off-by: Qingtang Zhou <qzhou@redhat.com>
qemu.test.cfg: Add usb-hub related test

Signed-off-by: Qingtang Zhou <qzhou@redhat.com>