qemu: fix hotplug of udp device with no connect host
Use an empty string to let qemu fill out the default.
This matches what's done in qemuBuildChrChardevStr.
https://bugzilla.redhat.com/show_bug.cgi?id=1454671
Signed-off-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: John Ferlan <jferlan@redhat.com>