qemu: Fix crash on incoming migration

In the first stage of incoming migration (qemuMigrationDstPrepareAny) we
call qemuMigrationEatCookie when there's no vm object created yet and
thus we don't have any private data to pass.

Broken by me in commit v5.6.0-109-gbf15b145ec.

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>
Reviewed-by: Erik Skultety <eskultet@redhat.com>

src/qemu/qemu_migration_cookie.c

@@ -1507,7 +1507,7 @@ qemuMigrationEatCookie(virQEMUDriverPtr driver,
     if (cookiein && cookieinlen &&
         qemuMigrationCookieXMLParseStr(mig,
                                        driver,
-                                       priv->qemuCaps,
+                                       priv ? priv->qemuCaps : NULL,
                                        cookiein,
                                        flags) < 0)
         goto error;