memimport: Support memfd blocks

memimport: Support memfd blocks

To transfer memfd-backed blocks without passing their fd every time,
thus minimizing overhead and avoiding fd leaks, a command is sent
with the memfd fd as ancil data very early on.

This command has an ID that uniquely identifies the memfd region.
Further memfd block references are then exclusively done using this
ID.

This commit implements the details of such 'permanent' mappings on
the receiving end, using memimport segments.

Suggested-by: David Henningsson <david.henningsson@canonical.com>
Signed-off-by: Ahmed S. Darwish <darwish.07@gmail.com>