fix >512-byte sector label/disk-test failure

fix >512-byte sector label/disk-test failure

* libparted/tests/disk.c: Include xstrtol.h.
(get_sector_size): New function: get sector size from the environment.
(create_disk): Use the new function to calculate a usable size,
rather than hard-coding the 512-byte-sector-specific size of 20MB.
* libparted/tests/common.c (_create_disk): Change meaning (and name)
of parameter to be size in bytes, not MiB.
Adjust callers.
* po/POTFILES.in: Add lib/xstrtol-error.c.