qemu: command: Introduce JSON variant of qemuBuildRomProps

Add a JSON variant of the generator 'rom' properties. For convenience
both the old and new are for now marked as unused, which will be removed
once the conversion is complete.

The formatted properties have following types according to QEMU.
'virtio-blk-pci' was used as an example:
  rombar=<uint32>        -  (default: 1)
  romfile=<str>

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>

src/qemu/qemu_command.c

@@ -1133,7 +1133,47 @@ qemuBuildVirtioDevStr(virBuffer *buf,
 }
 
 
-static int
+static int G_GNUC_UNUSED
+qemuBuildRomProps(virJSONValue *props,
+                  virDomainDeviceInfo *info)
+{
+    const char *romfile = NULL;
+    int rombar = -1;
+
+    if (info->romenabled == VIR_TRISTATE_BOOL_ABSENT &&
+        info->rombar == VIR_TRISTATE_SWITCH_ABSENT &&
+        !info->romfile)
+        return 0;
+
+    if (info->romenabled == VIR_TRISTATE_BOOL_NO) {
+        romfile = "";
+    } else {
+        romfile = info->romfile;
+
+        switch (info->rombar) {
+        case VIR_TRISTATE_SWITCH_OFF:
+            rombar = 0;
+            break;
+        case VIR_TRISTATE_SWITCH_ON:
+            rombar = 1;
+            break;
+        case VIR_TRISTATE_SWITCH_ABSENT:
+        case VIR_TRISTATE_SWITCH_LAST:
+            break;
+        }
+    }
+
+    if (virJSONValueObjectAdd(props,
+                              "k:rombar", rombar,
+                              "S:romfile", romfile,
+                              NULL) < 0)
+        return -1;
+
+    return 0;
+}
+
+
+static int G_GNUC_UNUSED
 qemuBuildRomStr(virBuffer *buf,
                 virDomainDeviceInfo *info)
 {