dracut-functions: get_persistent_dev() fall back to the original name

dracut-functions: get_persistent_dev() fall back to the original name

In case of raw disk/partition, ex. /dev/vda1, which doesn't contain any
filesystem on it. get_persistent_dev() would return empty. Now fix it to
return its original name, /dev/vda1 in above case. So that we don't have
to check its return string every time.

Signed-off-by: WANG Chao <chaowang@redhat.com>