Implicit controllers may be dependent on device definitions altered
in a post-parse callback. Specifically, if a console device is
defined without the target type, the type will be set in QEMU's
callback. In the case of s390, this is virtio, which requires
an implicit virtio-serial controller.
Signed-off-by: Viktor Mihajlovski <mihajlov@linux.vnet.ibm.com>
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