qemuDomainSetBlockIoTune: Skip monitor call for empty cdrom
qemuDomainSetBlockIoTune: Skip monitor call for empty cdrom

Similarly to startup of the VM qemu doesn't like setting throttling for
an empty drive. Just skip it since we do the correct thing once new
media is inserted.

Resolves: https://gitlab.com/libvirt/libvirt/-/issues/117
Signed-off-by: Peter Krempa <2cf5c04c61aa466e4a47bfedc747d17279c72ffc@redhat.com>
Reviewed-by: Ján Tomko <4cab11cfb98d3c937327354a78eb07dbb6ee2bc6@redhat.com>
Tested-by: Han Han <2f3eec96101b6aa65baff01c5b79fa5c74aa8195@redhat.com>

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