Load global config files by default

A new option, -nodefconfig is introduced to prevent loading from the default
config location.  Otherwise, two configuration files will be searched for,
qemu.conf and target-<TARGET_NAME>.conf.

To ensure that the default configuration is overridden by a user specified
config, we introduce a two stage option parsing mechanism.
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

qemu-options.hx

@@ -1965,6 +1965,15 @@ STEXI
 @item -writeconfig @var{file}
 Write device configuration to @var{file}.
 ETEXI
+DEF("nodefconfig", 0, QEMU_OPTION_nodefconfig,
+    "-nodefconfig\n"
+    "                do not load default config files at startup\n")
+STEXI
+@item -nodefconfig
+Normally QEMU loads a configuration file from @var{sysconfdir}/qemu.conf and
+@var{sysconfdir}/target-@var{ARCH}.conf on startup.  The @code{-nodefconfig}
+option will prevent QEMU from loading these configuration files at startup.
+ETEXI
 
 HXCOMM This is the last statement. Insert new options before this line!
 STEXI

vl.c

@@ -4730,6 +4730,7 @@ int main(int argc, char **argv, char **envp)
 #endif
     CPUState *env;
     int show_vnc_port = 0;
+    int defconfig = 1;
 
     init_clocks();
 
@@ -4789,6 +4790,44 @@ int main(int argc, char **argv, char **envp)
     tb_size = 0;
     autostart= 1;
 
+    /* first pass of option parsing */
+    optind = 1;
+    while (optind < argc) {
+        if (argv[optind][0] != '-') {
+            /* disk image */
+            continue;
+        } else {
+            const QEMUOption *popt;
+
+            popt = lookup_opt(argc, argv, &optarg, &optind);
+            switch (popt->index) {
+            case QEMU_OPTION_nodefconfig:
+                defconfig=0;
+                break;
+            }
+        }
+    }
+
+    if (defconfig) {
+        FILE *fp;
+        fp = fopen(CONFIG_QEMU_CONFDIR "/qemu.conf", "r");
+        if (fp) {
+            if (qemu_config_parse(fp) != 0) {
+                exit(1);
+            }
+            fclose(fp);
+        }
+
+        fp = fopen(CONFIG_QEMU_CONFDIR "/target-" TARGET_ARCH ".conf", "r");
+        if (fp) {
+            if (qemu_config_parse(fp) != 0) {
+                exit(1);
+            }
+            fclose(fp);
+        }
+    }
+
+    /* second pass of option parsing */
     optind = 1;
     for(;;) {
         if (optind >= argc)