qemu: Allow spaces in disk serial

https://bugzilla.redhat.com/show_bug.cgi?id=1195660

There's been a bug report appearing on the qemu-devel list, that
libvirt is unable to pass spaces in disk serial number [1]. Not only
our RNG schema forbids that, the code is not prepared either. However,
with a bit of escaping (if needed) we can allow spaces there.

1: https://lists.gnu.org/archive/html/qemu-devel/2015-02/msg04041.html

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>

docs/schemas/domaincommon.rng

@@ -5089,7 +5089,7 @@
   </define>
   <define name="diskSerial">
     <data type="string">
-      <param name="pattern">[A-Za-z0-9_\.\+\-]+</param>
+      <param name="pattern">[A-Za-z0-9_\.\+\- ]+</param>
     </data>
   </define>
   <define name="bridgeMode">