KVM: selftests: Print timer ctl register in ISTATUS assertion

Zenghui noted that the test assertion for the ISTATUS bit is printing
the current timer value instead of the control register in the case of
failure. While the assertion is sound, printing CNT isn't informative.

Change things around to actually print the CTL register value instead.
Reported-by: Zenghui Yu <yuzenghui@huawei.com>
Closes: https://lore.kernel.org/kvmarm/3188e6f1-f150-f7d0-6c2b-5b7608b0b012@huawei.com/Reviewed-by: Zenghui Yu <zenghui.yu@linux.dev>
Link: https://lore.kernel.org/r/20240212210932.3095265-2-oliver.upton@linux.devSigned-off-by: Oliver Upton <oliver.upton@linux.dev>

tools/testing/selftests/kvm/aarch64/arch_timer.c

@@ -160,7 +160,7 @@ static void guest_validate_irq(unsigned int intid,
 	__GUEST_ASSERT(xcnt >= cval,
 		       "xcnt = 0x%lx, cval = 0x%lx, xcnt_diff_us = 0x%lx",
 		       xcnt, cval, xcnt_diff_us);
-	__GUEST_ASSERT(xctl & CTL_ISTATUS, "xcnt = 0x%lx", xcnt);
+	__GUEST_ASSERT(xctl & CTL_ISTATUS, "xctl = 0x%lx", xctl);
 
 	WRITE_ONCE(shared_data->nr_iter, shared_data->nr_iter + 1);
 }