20_linux_xen: fix test for GRUB_DEVICE

Same fix as in 082bc9f7.

util/grub.d/20_linux_xen.in

@@ -47,7 +47,7 @@ esac
 # and mounting btrfs requires user space scanning, so force UUID in this case.
 if [ "x${GRUB_DEVICE_UUID}" = "x" ] || [ "x${GRUB_DISABLE_LINUX_UUID}" = "xtrue" ] \
     || ! test -e "/dev/disk/by-uuid/${GRUB_DEVICE_UUID}" \
-    || test -e "${GRUB_DEVICE}" && uses_abstraction "${GRUB_DEVICE}" lvm; then
+    || ( test -e "${GRUB_DEVICE}" && uses_abstraction "${GRUB_DEVICE}" lvm ); then
   LINUX_ROOT_DEVICE=${GRUB_DEVICE}
 else
   LINUX_ROOT_DEVICE=UUID=${GRUB_DEVICE_UUID}