qemu: migration: Don't attempt to fall back to old-style storage migration

qemu: migration: Don't attempt to fall back to old-style storage migration

QEMU supported the NBD server required for the new-style migration for a
long time already and when coupled with -blockdev the old style
migration doesn't even work, thus remove support for it.

This patch modifies the code to check that the destination returned data
for the NBD migration and returns an error if it did not and deletes the
fallback code paths which would not work.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>