During virStorageBackendDiskMakeDataVol processing, if we find an extended
partition, then handle it specially when updating the capacity/allocation
rather than calling virStorageBackendUpdateVolInfo.

As it turns out, once a logical partition exists, any attempt to refresh
the pool or after libvirtd restart/reload will result in a failure to open
the extended partition device resulting in the inability to start the pool.
The downside to this is we will lose the <permissions> and <timestamps> for
the extended partition upon subsequent restart, refresh, reload since the
stat() in virStorageBackendUpdateVolTargetInfoFD will not be called. However,
since it's really only a container and shouldn't directly be used for
storage that seems reasonable.

Therefore, only use the existing code that already had a comment about
getting the allocation wrong for extended partitions for just the setting
of the extended partition data.