qemu.tests.usb_storage: Fix regex string for check serial/removable option

Use '\s' to match any whitespace character, like: [ \n\r].

Signed-off-by: Shuping Cui <scui@redhat.com>
Merge pull request #42 from spcui/usb_storage_fix

qemu.tests.usb_storage: Fix regex string for check serial/removable option