disk storage: fix allocation size for pool format dos

The changed condition was always false because the function was always
called with boundary values 0.

Use the free extent's start value to get its start offset from the
cylinder boundary and determine if the needed size for allocation
needs to be expanded too in case the offset doesn't fit within extra
bytes for alignment.

This fixes an issue where vol-create-from will call qemu-img convert
to create a destination volume of same capacity as the source volume
and qemu-img will error 'Cannot grow device files' due to the partition
being too small for the source although both destination partition and
source volume have the same capacity.

Signed-off-by: Sebastian Mitterle <smitterl@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>

src/storage/storage_backend_disk.c

@@ -691,7 +691,7 @@ virStorageBackendDiskPartBoundaries(virStoragePoolObjPtr pool,
          if (def->source.format == VIR_STORAGE_POOL_DISK_DOS) {
              /* align to cylinder boundary */
              neededSize += extraBytes;
-             if ((*start % cylinderSize) > extraBytes) {
+             if ((dev->freeExtents[i].start % cylinderSize) > extraBytes) {
                  /* add an extra cylinder if the offset can't fit within
                     the extra bytes we have */
                  neededSize += cylinderSize;