libfstools: ubi: rework reading volumes

libfstools: ubi: rework reading volumes

UBI volume IDs don't have to be contiguous, this may happen if there
were few and the not last one was removed. In such case we may have
e.g. ubi0_0, ubi0_2.
It means we can't simply read value from "volumes_count" and iterate
from 0 to the count - 1.

This patch rewrites ubi_part_match to read whole directory and look
for ubi%d_%d entries in it.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>