[PATCH for-9.1?] system/vl.c: Print machine name, not "(null)", for unknown machine types

[Peter Maydell]

In commit 412d294ffdc we tried to improve the error message printed when
the machine type is unknown, but we used the wrong variable, resulting in:

$ ./build/x86/qemu-system-aarch64 -M bang
qemu-system-aarch64: unsupported machine type: "(null)"
Use -machine help to list supported machines

Use the right variable, so we produce more helpful output:

$ ./build/x86/qemu-system-aarch64 -M bang
qemu-system-aarch64: unsupported machine type: "bang"
Use -machine help to list supported machines

Note that we must move the qdict_del() to below the error_setg(),
because machine_type points into the value of that qdict entry,
and deleting it will make the pointer invalid.

Cc: qemu-stable@nongnu.org
Fixes: 412d294ffdc ("vl.c: select_machine(): add selected machine type to error 
message")
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
---
Tagged as for-9.1 because this is a sort-of regression since 9.0
(in 9.0 we didn't try to print the machine type name at all in
this error). But it's also very close to release and this is only
cosmetics in an error message. The cc-stable is for the case where
we don't put it into 9.1 and then backport it to the 9.1 stable
branch after release.

 system/vl.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/system/vl.c b/system/vl.c
index 41d53d2456a..01b8b8e77ad 100644
--- a/system/vl.c
+++ b/system/vl.c
@@ -1679,10 +1679,10 @@ static MachineClass *select_machine(QDict *qdict, Error 
**errp)
 
     if (machine_type) {
         machine_class = find_machine(machine_type, machines);
-        qdict_del(qdict, "type");
         if (!machine_class) {
-            error_setg(errp, "unsupported machine type: \"%s\"", optarg);
+            error_setg(errp, "unsupported machine type: \"%s\"", machine_type);
         }
+        qdict_del(qdict, "type");
     } else {
         machine_class = find_default_machine(machines);
         if (!machine_class) {
-- 
2.34.1