qemu.tests: Add a test to check cpu load with/without usb device

Signed-off-by: Qingtang Zhou <qzhou@redhat.com>
qemu.tests: Add negative test for usb host

Signed-off-by: Qingtang Zhou <qzhou@redhat.com>
qemu.tests: Make usb host test can repeat multiple times

Signed-off-by: Qingtang Zhou <qzhou@redhat.com>
qemu.tests: Add a test for checking isobufs option for usb

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