migration: Extract code to mark all parameters as present

MigrationParameters needs to have all of its has_* fields marked as
true when used as the return of query_migrate_parameters because the
corresponding QMP command has all of its members non-optional by
design, despite them being marked as optional in migration.json.

Extract this code into a function and make it assert if any field is
missing. With this we ensure future changes will not inadvertently
leave any parameters missing.

Note that the block-bitmap-mapping is a special case because the empty
list is considered a valid value, so it has historically not been
present in the command's output if it has never been set.

CC: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>
Link: https://lore.kernel.org/r/20251215220041.12657-13-farosas@suse.de
Signed-off-by: Peter Xu <peterx@redhat.com>