Handle all-numeric serial numbers

If a device has a serial number with no alpha or other non-numeric values, it'll be
interpreted as an integer in the extractvars() function.  Since integers don't have
a .strip() method, lsdrv throws an attribute error when trying to format the value.

Reported by: Johannes Truschnigg <johannes.truschnigg@geizhals.at>
Submitted by: Johannes Truschnigg <johannes.truschnigg@geizhals.at>