virsysinfo: Parse OEM strings

Setting OEM strings for a domain was introduced in
v4.1.0-rc1~315. However, any application that wanted to use them
(e.g. to point to an URL where a config file is stored) had to
'dmidecode -u --oem-string N' (where N is index of the string).
Well, we can expose them under our <sysinfo/> XML and if the
domain is running Libvirt inside it can be obtained using
virConnectGetSysinfo() API.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>

tests/sysinfodata/x86sysinfo.data

@@ -81,3 +81,9 @@ Memory Device
 	Serial Number: 29057112
 	Asset Tag: 0839
 	Part Number: IMSH2GS13A1F1C-10F
+
+OEM Strings
+        String 1: Hello
+        String 2: World
+        String 3: Ha ha ha try parsing\n.      String 3: this correctly.      String 4:then
+        String 4: This is, more tricky value=escaped

tests/sysinfodata/x86sysinfo.expect

@@ -50,4 +50,12 @@
     <entry name='serial_number'>29057112</entry>
     <entry name='part_number'>IMSH2GS13A1F1C-10F</entry>
   </memory_device>
+  <oemStrings>
+    <entry>Hello</entry>
+    <entry>World</entry>
+    <entry>Ha ha ha try parsing\n
+      String 3: this correctly
+      String 4:then</entry>
+    <entry>This is, more tricky value=escaped</entry>
+  </oemStrings>
 </sysinfo>