qapi: Use QAPI_LIST_PREPEND() where possible

Anywhere we create a list of just one item or by prepending items
(typically because order doesn't matter), we can use
QAPI_LIST_PREPEND().  But places where we must keep the list in order
by appending remain open-coded until later patches.

Note that as a side effect, this also performs a cleanup of two minor
issues in qga/commands-posix.c: the old code was performing
 new = g_malloc0(sizeof(*ret));
which 1) is confusing because you have to verify whether 'new' and
'ret' are variables with the same type, and 2) would conflict with C++
compilation (not an actual problem for this file, but makes
copy-and-paste harder).

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20201113011340.463563-5-eblake@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
[Straightforward conflicts due to commit a8aa94b5f8 "qga: update
schema for guest-get-disks 'dependents' field" and commit a10b453a52
"target/mips: Move mips_cpu_add_definition() from helper.c to cpu.c"
resolved.  Commit message tweaked.]
Signed-off-by: Markus Armbruster <armbru@redhat.com>
diff --git a/qga/commands-posix-ssh.c b/qga/commands-posix-ssh.c
index 749167e..2dda136 100644
--- a/qga/commands-posix-ssh.c
+++ b/qga/commands-posix-ssh.c
@@ -293,17 +293,12 @@
 
     ret = g_new0(GuestAuthorizedKeys, 1);
     for (i = 0; authkeys[i] != NULL; i++) {
-        strList *new;
-
         g_strstrip(authkeys[i]);
         if (!authkeys[i][0] || authkeys[i][0] == '#') {
             continue;
         }
 
-        new = g_new0(strList, 1);
-        new->value = g_strdup(authkeys[i]);
-        new->next = ret->keys;
-        ret->keys = new;
+        QAPI_LIST_PREPEND(ret->keys, g_strdup(authkeys[i]));
     }
 
     return g_steal_pointer(&ret);