Don't try to instantiate a default NIC if it is not available (since
this will cause QEMU to abort). Emit a warning instead.

Message-Id: <20230512124033.502654-5-thuth@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>