qemu.tests.cfg: Enable qemu guest agent test for RHEL guest

Signed-off-by: Qingtang Zhou <qzhou@redhat.com>
qemu.tests.cfg: Disable xhci on old windows for RHEL host

Signed-off-by: Qingtang Zhou <qzhou@redhat.com>
qemu.tests.cfg: Move usb_(re)boot config for win guest to Windows.cfg

These parameters would be overridden by the parameters in
each usb device config block. So move it into guest-os's
config files.

Signed-off-by: Qingtang Zhou <qzhou@redhat.com>
Merge pull request #638 from zhouqt/minor-fix

qemu: Minor fixes for qemu test cases