qemuDomainChangeEjectableMedia: Don't always remove managed PR daemon

qemuDomainChangeEjectableMedia: Don't always remove managed PR daemon

When changing media we'd attempt to remove the managed pr daemon even if
neither of the images involved in the media change used it. This caused
libvirtd to log a spurious error:

2020-03-18 01:41:19.832+0000: 643207: error : qemuMonitorJSONCheckError:412 : internal error: unable to execute QEMU command 'object-del': object 'pr-helper0' not found

With this patch we completely avoid calling the deletion code.

https://bugzilla.redhat.com/show_bug.cgi?id=1814486

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>