virQEMUCapsHasPCIMultiBus: use def->os.arch

We do not fill out qemuCaps->arch when parsing status XML.

Use def->os.arch like we do for PPC.

This fixes hotplug after daemon restart for domains that use
a user alias for the implicit pci-root on x86.

https://bugzilla.redhat.com/show_bug.cgi?id=1518148

tests/qemuhotplugtestcpus/ppc64-modern-bulk-result-live.xml

@@ -56,7 +56,7 @@
       <address type='pci' domain='0x0000' bus='0x00' slot='0x01' function='0x2'/>
     </controller>
     <controller type='pci' index='0' model='pci-root'>
-      <alias name='pci'/>
+      <alias name='pci.0'/>
     </controller>
     <input type='mouse' bus='ps2'>
       <alias name='input0'/>