When --direct is used when migrating a domain running on a hypervisor
that does not support direct migration (such as QEMU), the caller would
get the following error message:

    this function is not supported by the connection driver:
    virDomainMigrateToURI2

which is a complete nonsense since qemu driver implements
virDomainMigrateToURI2. This patch would emit a more sensible error in
this case:

    Requested operation is not valid: direct migration is not supported
    by the connection driver