qemu.tests: Small bugs fix in format_disk

Sometimes the output include some specific control character, like: ^H,
so update check content method.

Signed-off-by: Shuping Cui <scui@redhat.com>
qemu.tests.cfg: Update usb_sleep_time config for usb.cfg

usb device verify need some time to let the guest os
to init the device after hotpluging.

Signed-off-by: Shuping Cui <scui@redhat.com>