The new qemu-pr-helper (commit b855f8d1) should not be checked in,
even when doing in-tree builds.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20170926151421.14557-1-eblake@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>