KVM: x86/xen: Fix lockdep warning on "recursive" gpc locking

In commit 5ec3289b ("KVM: x86/xen: Compatibility fixes for shared runstate
area") we declared it safe to obtain two gfn_to_pfn_cache locks at the same
time:
	/*
	 * The guest's runstate_info is split across two pages and we
	 * need to hold and validate both GPCs simultaneously. We can
	 * declare a lock ordering GPC1 > GPC2 because nothing else
	 * takes them more than one at a time.
	 */

However, we forgot to tell lockdep. Do so, by setting a subclass on the
first lock before taking the second.

Fixes: 5ec3289b

 ("KVM: x86/xen: Compatibility fixes for shared runstate area")
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Message-Id: <20230111180651.14394-1-dwmw2@infradead.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>