CP-9052: add support for large VHDs in td-util

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>
CP-9052: initialise variable

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>
CP-9052: print error message on snapshot failure

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>
CP-9052: get parent version before checking file limits

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>
CP-9052: work on tapdisk VHD driver (incomplete)

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>
CP-9052: more on unit tests

Signed-off-by: Thanos Makatos <thanos.makatos@citrix.com>