migration: Fix migrate-set-parameters argument validation

Commit 741d4086 "migration: Use proper types in json" (v2.12.0)
switched MigrationParameters to narrower integer types, and removed
the simplified qmp_migrate_set_parameters()'s argument checking
accordingly.

Good idea, except qmp_migrate_set_parameters() takes
MigrateSetParameters, not MigrationParameters.  Its job is updating
migrate_get_current()->parameters (which *is* of type
MigrationParameters) according to its argument.  The integers now get
truncated silently.  Reproducer:

    ---> {'execute': 'query-migrate-parameters'}
    <--- {"return": {[...] "compress-threads": 8, [...]}}
    ---> {"execute": "migrate-set-parameters", "arguments": {"compress-threads": 257}}
    <--- {"return": {}}
    ---> {'execute': 'query-migrate-parameters'}
    <--- {"return": {[...] "compress-threads": 1, [...]}}

Fix by resynchronizing MigrateSetParameters with MigrationParameters.

Fixes: 741d4086
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-Id: <20210202141734.2488076-2-armbru@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>