qemu: Do not erase duplicate devices from namespace if error occurs

qemu: Do not erase duplicate devices from namespace if error occurs

If the attempt to attach a device failed, we erased the
unattached device from the namespace. This resulted in erasing an
already attached device in case of a duplicate. We need to check
for existing file in the namespace in order to determine erasing
it in case of a failure.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1780508

Signed-off-by: Kristina Hanicova <khanicov@redhat.com>
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>