block/vhdx.c: Mark parent_vhdx_guid variable as unused

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.

Change-Id: I9748b3b20922666e5f2efceb80e37e99ea3430ce
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>