Add the ability to pass arbitrary fds to cancellable subprocesses

Based on 2b2e0fb51917d59e568657b7eda26444d4907c6b in xen-api.git

Signed-off-by: Jon Ludlam <jonathan.ludlam@citrix.com>
CP-8784: Use new suspend-image format

Detect older formats and pipe through conversion tool. Newer formats have
a proper framing protocol.

Signed-off-by: Si Beaumont <simon.beaumont@citrix.com>
Ported-by: Jon Ludlam <jonathan.ludlam@citrix.com>