Use qemuDomainAttachDeviceDiskLive to change the media in
qemuDomainChangeDiskLive as the former function already does all the
necessary steps to prepare the new medium.

This also allows us to turn qemuDomainChangeEjectableMedia static.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NJán Tomko <jtomko@redhat.com>