v2v: tests: isolate SHA1 calculation in an own shared function

sha1sum(1) does not exist everywhere, so wrap it in an own function so
the right implementation can be chosen on each OS.  Also, wrapping it
avoid using awk everytime.

test-data/guestfs-hashsums.sh

@@ -28,3 +28,16 @@ do_md5 ()
       ;;
   esac
 }
+
+do_sha1 ()
+{
+  case "$(uname)" in
+    Linux)
+      sha1sum "$1" | awk '{print $1}'
+      ;;
+    *)
+      echo "$0: unknown method to calculate SHA1 of file on $(uname)"
+      exit 1
+      ;;
+  esac
+}