qemu.tests.cfg: Update usb tests config

* Disable usb_nodev and usb_host on RHEL6 host
* Limit usb_audio to RHEL7+ host.
* Use human monitor for usb_hotplug test.
* Limit usb_hotplug test to RHEL6+ guest.

Signed-off-by: Qingtang Zhou <qzhou@redhat.com>
Acked-by: Feng Yang <fyang@redhat.com>
qemu.tests.usb_storage: Get block device real path before using it

Some guests (eg, RHEL4.9) can't mount the usb storage device which
path contains quotation mark, thus this patch make test script
try to get the real path before using it.

Signed-off-by: Qingtang Zhou <qzhou@redhat.com>
Acked-by: Suqin Huang <shuang@redhat.com>
qemu.tests.cfg.usb: Disable min/opt_io_size checking in RHEL3/4/5

These guests don't support these kernel option, so disable them
directly.

Signed-off-by: Qingtang Zhou <qzhou@redhat.com>
Acked-by: Xiaoqing Wei <xwei@redhat.com>