Revert unnecesary lcase in ds-identify (#978)

This patch reverts an unnecessary lcase optimization in the
ds-identify script. SystemD documents the values produced by
the systemd-detect-virt command are lower case, and the mapping
table used by the FreeBSD check is also lower-case.

The optimization added two new forked processes, needlessly
causing overhead.

tools/ds-identify

@@ -449,7 +449,7 @@ detect_virt() {
 read_virt() {
     cached "$DI_VIRT" && return 0
     detect_virt
-    DI_VIRT="$(echo "${_RET}" | tr '[:upper:]' '[:lower:]')"
+    DI_VIRT="${_RET}"
 }
 
 is_container() {