m4/virt-loader-nvram: use quotation for list of loader:nvram pairs

The bug was introduced by commit 08c2d148.  The string must be quoted
because it is used as function argument.
Signed-off-by: NPavel Hrdina <phrdina@redhat.com>

m4/virt-loader-nvram.m4

@@ -31,7 +31,7 @@ AC_DEFUN([LIBVIRT_CHECK_LOADER_NVRAM], [
     if test $(expr $l % 2) -ne 0 ; then
       AC_MSG_ERROR([Malformed --with-loader-nvram argument])
     fi
-    AC_DEFINE_UNQUOTED([DEFAULT_LOADER_NVRAM], [$with_loader_nvram],
+    AC_DEFINE_UNQUOTED([DEFAULT_LOADER_NVRAM], ["$with_loader_nvram"],
                        [List of loader:nvram pairs])
   fi
 ])