Also enforce the enum type for MigrateSetMaxSpeed and MigrateGetMaxSpeed
functions which previously accepted generic uint32 type.
Technically, changing the flags parameter type to a named enum type is
considered an API incompatible change, but in practice the flags
parameter was unused in the past, so apps should have been passing an
untyped '0' - Go will take care of type inference here, so there should
be no breakage within apps.

Signed-off-by: Erik Skultety <eskultet@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>