qemu: fix hotplug of udp device with no connect host
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 <4cab11cfb98d3c937327354a78eb07dbb6ee2bc6@redhat.com>
Reviewed-by: John Ferlan <87558058f6f829e5ec976c8ef960720af4ff9c7d@redhat.com>

libvirt/libvirt,crobinso/libvirt,andreabolognani/libvirt,nertpinx/libvirt,jfehlig/libvirt,fabianfreyer/libvirt,jardasgit/libvirt,libvirt/libvirt,jardasgit/libvirt,nertpinx/libvirt,eskultety/libvirt,jardasgit/libvirt,crobinso/libvirt,crobinso/libvirt,nertpinx/libvirt,zippy2/libvirt,jfehlig/libvirt,andreabolognani/libvirt,zippy2/libvirt,libvirt/libvirt,fabianfreyer/libvirt,nertpinx/libvirt,zippy2/libvirt,fabianfreyer/libvirt,olafhering/libvirt,crobinso/libvirt,andreabolognani/libvirt,fabianfreyer/libvirt,libvirt/libvirt,eskultety/libvirt,olafhering/libvirt,jfehlig/libvirt,andreabolognani/libvirt,zippy2/libvirt,olafhering/libvirt,nertpinx/libvirt,jardasgit/libvirt,andreabolognani/libvirt,jardasgit/libvirt,fabianfreyer/libvirt,eskultety/libvirt,jfehlig/libvirt,eskultety/libvirt,eskultety/libvirt,olafhering/libvirt