qemu: Expose availability of PS/2 feature in domcaps

This advertises the feature only for the architectures and
machine types where it can actually be used.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>

tests/domaincapsdata/qemu_9.1.0.x86_64.xml

@@ -276,6 +276,7 @@
     <backingStoreInput supported='yes'/>
     <backup supported='yes'/>
     <async-teardown supported='yes'/>
+    <ps2 supported='yes'/>
     <sev supported='no'/>
     <sgx supported='no'/>
     <hyperv supported='yes'>