block/gluster: update .help of BLOCK_OPT_PREALLOC option

Add missing 'falloc' among the allowed values of 'preallocation'
option; show it and 'full' only when they are supported.
('falloc' is supported if defined CONFIG_GLUSTERFS_FALLOCATE,
'full' is supported if defined CONFIG_GLUSTERFS_ZEROFILL)

Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20190524075848.23781-4-sgarzare@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

block/gluster.c

@@ -98,7 +98,14 @@ static QemuOptsList qemu_gluster_create_opts = {
         {
             .name = BLOCK_OPT_PREALLOC,
             .type = QEMU_OPT_STRING,
-            .help = "Preallocation mode (allowed values: off, full)"
+            .help = "Preallocation mode (allowed values: off"
+#ifdef CONFIG_GLUSTERFS_FALLOCATE
+                    ", falloc"
+#endif
+#ifdef CONFIG_GLUSTERFS_ZEROFILL
+                    ", full"
+#endif
+                    ")"
         },
         {
             .name = GLUSTER_OPT_DEBUG,