Commit f548480b broke migration v3 on qemu, because the driver
passed flags on through to qemu_migration even though
qemu_migration wasn't using those flags.

* src/qemu/qemu_migration.h (QEMU_MIGRATION_FLAGS): New define.
* src/qemu/qemu_driver.c: Simplify all migration callbacks.
* src/qemu/qemu_migration.c (qemuMigrationConfirm): Fix regression.