cmd/vrun: Use exec.LookPath

Use exec.LookPath to find the binary to execute. So we can run "vrun
bash" instead of "vrun /bin/bash".

Change-Id: I7153c0057f7ea416f0d04c5c918cadcec544548f
diff --git a/cmd/vrun/vrun.go b/cmd/vrun/vrun.go
index e604d9f..4bdafd1 100644
--- a/cmd/vrun/vrun.go
+++ b/cmd/vrun/vrun.go
@@ -6,6 +6,7 @@
 
 import (
 	"os"
+	"os/exec"
 	"path/filepath"
 	"syscall"
 	"time"
@@ -126,7 +127,12 @@
 		}
 		conn.Close()
 	}
-	err := syscall.Exec(cmd[0], cmd, os.Environ())
+	p, err := exec.LookPath(cmd[0])
+	if err != nil {
+		vlog.Errorf("Couldn't find %q", cmd[0])
+		return err
+	}
+	err = syscall.Exec(p, cmd, os.Environ())
 	vlog.Errorf("Couldn't exec %s.", cmd[0])
 	return err
 }