cgs->ready can be false if the accelerator does not look at
current_machine->cgs altogether.

Assume that the lack of initialization is due to this, and
report a nicer error instead of an assertion failure:

    $ qemu-system-x86_64 -object sev-guest,id=sev0,policy=0x5,id=sev0,cbitpos=51,reduced-phys-bits=1 -M confidential-guest-support=sev0
    qemu-system-x86_64: ../softmmu/vl.c:2619: qemu_machine_creation_done: Assertion `machine->cgs->ready' failed.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 system/vl.c | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

diff --git a/system/vl.c b/system/vl.c
index 0c970cf0203..c6442229824 100644
--- a/system/vl.c
+++ b/system/vl.c
@@ -2676,11 +2676,10 @@ static bool qemu_machine_creation_done(Error **errp)
 
     qdev_machine_creation_done();
 
-    if (machine->cgs) {
-        /*
-         * Verify that Confidential Guest Support has actually been initialized
-         */
-        assert(machine->cgs->ready);
+    if (machine->cgs && !machine->cgs->ready) {
+        error_setg(errp, "accelerator does not support confidential guest %s",
+                   object_get_typename(OBJECT(machine->cgs)));
+        exit(1);
     }
 
     if (foreach_device_config(DEV_GDB, gdbserver_start) < 0) {
-- 
2.44.0