block/vhdx.c: Mark parent_vhdx_guid variable as unused

The parent_vhdx_guid variable is defined but never used, which provokes
complaints from newer versions of clang. Since the variable definition
is here acting as documentation of the image format, mark it with the
'unused' attribute to keep the compiler happy rather than simply
deleting it.
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Reviewed-by: NFam Zheng <famz@redhat.com>
Reviewed-by: NJeff Cody <jcody@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

block/vhdx.c

@@ -99,7 +99,8 @@ static const MSGUID logical_sector_guid = { .data1 = 0x8141bf1d,
 /* Each parent type must have a valid GUID; this is for parent images
  * of type 'VHDX'.  If we were to allow e.g. a QCOW2 parent, we would
  * need to make up our own QCOW2 GUID type */
-static const MSGUID parent_vhdx_guid = { .data1 = 0xb04aefb7,
+static const MSGUID parent_vhdx_guid __attribute__((unused))
+                                     = { .data1 = 0xb04aefb7,
                                          .data2 = 0xd19e,
                                          .data3 = 0x4a81,
                                          .data4 = { 0xb7, 0x89, 0x25, 0xb8,