ci: update qemu prebuilt to version 6.0.0.

Updated prebuilt hash.
Updated qemu command line with:
-no-reboot: Qemu implements a "secure GPIO" controller for the machine
reboot/shutdown [1]. The upstream TF-A Qemu platform port uses this
control by default (rather than semihosting formerly). The shutdown
control in Qemu inadvertently triggers a reboot as explained in [2].
Workaround it by letting Qemu perform a shutdown instead of reboot
when the shutdown gpio control is triggered.
-Suppress a warning by using the recommended -machine secure=on option.

[1] https://github.com/qemu/qemu/commit/daa726d92604b72650d86ada01935e03a909d4dd
[2] https://review.trustedfirmware.org/c/TF-A/trusted-firmware-a/+/8093

Change-Id: I1b814cbe9e32e5f2ded463520d80ff425cbdcd0a
Signed-off-by: Olivier Deprez <olivier.deprez@arm.com>
diff --git a/prebuilts b/prebuilts
index 71a6c24..40af98f 160000
--- a/prebuilts
+++ b/prebuilts
@@ -1 +1 @@
-Subproject commit 71a6c2457e5b2ee89a0e1a9dc1eb30333e19c402
+Subproject commit 40af98f0b95026b98e34471558c04cfae7129d8b
diff --git a/test/hftest/hftest.py b/test/hftest/hftest.py
index 3328a5c..359c071 100755
--- a/test/hftest/hftest.py
+++ b/test/hftest/hftest.py
@@ -224,7 +224,7 @@
         exec_args = [
             "timeout", "--foreground", time_limit,
             os.path.abspath("prebuilts/linux-x64/qemu/qemu-system-aarch64"),
-            "-machine", "virt,virtualization=on,gic-version=3",
+            "-no-reboot", "-machine", "virt,virtualization=on,gic-version=3",
             "-cpu", cpu, "-smp", "4", "-m", "1G",
             "-nographic", "-nodefaults", "-serial", "stdio",
             "-d", "unimp", "-kernel", os.path.abspath(self.args.hypervisor),
@@ -237,7 +237,7 @@
             exec_args += ["-bios",
                 os.path.abspath(bl1_path),
                 "-machine", "secure=on", "-semihosting-config",
-                "enable,target=native"]
+                "enable=on,target=native"]
 
         if self.args.initrd:
             exec_args += ["-initrd", os.path.abspath(self.args.initrd)]