libxl: Temporarily hardcode paths to ovmf firmwares

This is a quick, hacky fix for bsc#1159793 until there is time to work on
upstream support for firmware autoselection in the xen driver. Sadly, the
upstream efforts to improve firmware handling in the qemu driver broke
the firmware handling in the xen driver.

Index: libvirt-6.3.0/src/libxl/libxl_conf.c
===================================================================
--- libvirt-6.3.0.orig/src/libxl/libxl_conf.c
+++ libvirt-6.3.0/src/libxl/libxl_conf.c
@@ -1777,6 +1777,15 @@ libxlDriverConfigNew(void)
     cfg->autoDumpDir = g_strdup(LIBXL_DUMP_DIR);
     cfg->channelDir = g_strdup(LIBXL_CHANNEL_DIR);
 
+    /* Begin hack ---
+     * bsc#1159793: Until there is time to work on proper upstream support for
+     * firmware autoselection in the xen driver we'll go with this minimal fix.
+     */
+#define DEFAULT_LOADER_NVRAM "/usr/share/qemu/ovmf-x86_64-ms-4m.bin:/usr/share/qemu/ovmf-x86_64-ms-4m-vars.bin:/usr/share/qemu/ovmf-x86_64-ms.bin:/usr/share/qemu/ovmf-x86_64-ms-vars.bin"
+    /*
+     * --- End hack
+     */
+
 #ifdef DEFAULT_LOADER_NVRAM
     if (virFirmwareParseList(DEFAULT_LOADER_NVRAM,
                              &cfg->firmwares,