virt.virttest.storage:avoid the wrong to raise an error

Currently, it will rase and error when '/dev/sdaa' match the pattern
of the blacklist. This should not be happen as we just want sda sda1
and so on to be in the blacklist.

add one more check to avoid this error.

Signed-off-by: Mike Qiu<qiudayu@linux.vnet.ibm.com>