QemuOpts: Convert qemu_opt_set_number() to Error, fix its use

Return the Error object instead of reporting it with
qerror_report_err().

Change callers that assume the function can't fail to pass
&error_abort, so that should the assumption ever break, it'll break
noisily.

Turns out all callers outside its unit test assume that.  We could
drop the Error ** argument, but that would make the interface less
regular, so don't.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
diff --git a/vl.c b/vl.c
index 27e738a..8ea8b08 100644
--- a/vl.c
+++ b/vl.c
@@ -2684,7 +2684,7 @@
     }
 
     /* store value for the future use */
-    qemu_opt_set_number(opts, "size", ram_size);
+    qemu_opt_set_number(opts, "size", ram_size, &error_abort);
     *maxram_size = ram_size;
 
     maxmem_str = qemu_opt_get(opts, "maxmem");