Disk manufacturers are fond of quoting sizes in powers of 10,
rather than powers of 2 (after all, 2.1 GB sounds larger than
2.0 GiB, even though the exact opposite is true).  So, we might
as well follow coreutils' lead in supporting three types of
suffix: single letter ${u} (which we already had) and ${u}iB
for the power of 2, and ${u}B for power of 10.

Additionally, it is impossible to create a file with more than
2**63 bytes, since off_t is signed (if you have enough storage
to even create one 8EiB file, I'm jealous).  This now reports
failure up front rather than down the road when the kernel
finally refuses an impossible size.

* docs/schemas/basictypes.rng (unit): Add suffixes.
* src/conf/storage_conf.c (virStorageSize): Use new function.
* docs/formatstorage.html.in: Document it.
* tests/storagevolxml2xmlin/vol-file-backing.xml: Test it.
* tests/storagevolxml2xmlin/vol-file.xml: Likewise.