qemu: Properly handle setting of <iotune> for empty cdrom

qemu: Properly handle setting of <iotune> for empty cdrom

When starting a VM with an empty cdrom which has <iotune> configured the
startup fails as qemu is not happy about setting tuning for an empty
drive:

 error: internal error: unable to execute 'block_set_io_throttle', unexpected error: 'Device has no medium'

Resolve this by skipping the setting of throttling for empty drives and
updating the throttling when new medium is inserted into the drive.

Resolves: https://gitlab.com/libvirt/libvirt/-/issues/111
Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>