qemu: cold-plug of watchdog
qemu: cold-plug of watchdog

https://bugzilla.redhat.com/show_bug.cgi?id=1447169

With this patch users can cold plug a watchdog. Things are pretty
simple because a domain can have at most one watchdog device.

Signed-off-by: Michal Privoznik <83d82aaba2eed257f4814b0c239c260c4caaadf0@redhat.com>

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