VM prepare image reserved fields usage

VM prepare image reserved fields usage

The version 4 image format did not write 0 into reserved_* fields (#2580).
Therefore current version 4 images are not forward compatible with image versions that use these fields.
By introducing an escape code 0 into the data_size field,
three reserved_* fields now become available for redefinition, while keeping image version at 4.

This version can transparently read regular v4 image files, as well as ones with h.data_size=0 and h.escaped_data_size=data_size