qemu.tests.cfg: Enable usb_nodev variant on RHEL host

Signed-off-by: Qingtang Zhou <qzhou@redhat.com>
shared.cfg: Fix the s4_support_chk_cmd for Fedora guest

Signed-off-by: Qingtang Zhou <qzhou@redhat.com>
Merge pull request #616 from zhouqt/minor-fix

Qemu: Minor fix for some config of qemu test cases