qemu: Fix NULL ptr dereference caused by qemuDomainDefFormatBufInternal

qemuDomainDefFormatBufInternal function wasn't testing whether the CPU
was actually defined in the XML and saving such a domain resulted in the
following backtrace:

0 in qemuDomainMakeCPUMigratable (cpu=0x0)
1 in qemuDomainDefFormatBufInternal()
2 in qemuDomainDefFormatXMLInternal()
3 in qemuDomainDefFormatLive()
4 in qemuDomainSaveInternal()
5 in qemuDomainSaveFlags()
6 in qemuDomainSave()
7 in virDomainSave()
Signed-off-by: NErik Skultety <eskultet@redhat.com>
Reviewed-by: NDaniel Henrique Barboza <danielhb413@gmail.com>

src/qemu/qemu_domain.c

@@ -9105,7 +9105,7 @@ qemuDomainDefFormatBufInternal(virQEMUDriverPtr driver,
                 goto cleanup;
         }
 
-        if (qemuDomainMakeCPUMigratable(def->cpu) < 0)
+        if (def->cpu && qemuDomainMakeCPUMigratable(def->cpu) < 0)
             goto cleanup;
     }