qemu: Add support for setting downtime-limit migration parameter
We already support setting the maximum downtime with a dedicated
virDomainMigrateSetMaxDowntime API. This patch does not implement
another way of setting the downtime by adding a new public migration
parameter. It just makes sure any parameter we are able to get from a
QEMU monitor by query-migrate-parameters can be passed back to QEMU via
migrate-set-parameters.
Signed-off-by: Jiri Denemark <jdenemar@redhat.com> Reviewed-by: John Ferlan <jferlan@redhat.com>