qemu: do not revert to NULL bandwidth
qemu: do not revert to NULL bandwidth

Otherwise an attempt to set an invalid value:
  virsh domiftune rhel8.2 vnet0 --outbound 4294968
on an interface with no bandwidth set crashes.

Signed-off-by: Ján Tomko <4cab11cfb98d3c937327354a78eb07dbb6ee2bc6@redhat.com>
Fixes: f02e21cb3379a41cd42f2d8116f2d10dabace83b
https://bugzilla.redhat.com/show_bug.cgi?id=1800505
Reviewed-by: Daniel Henrique Barboza <627b089ad62deae9aaf06ccfa2c9056df28927e0@gmail.com>

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