xen: support GSI -> pirq remapping in PV on HVM guests

Disable pcifront when running on HVM: it is meant to be used with pv
guests that don't have PCI bus.

Use acpi_register_gsi_xen_hvm to remap GSIs into pirqs.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
diff --git a/include/xen/interface/features.h b/include/xen/interface/features.h
index 70d2563..b6ca39a 100644
--- a/include/xen/interface/features.h
+++ b/include/xen/interface/features.h
@@ -47,6 +47,9 @@
 /* x86: pvclock algorithm is safe to use on HVM */
 #define XENFEAT_hvm_safe_pvclock           9
 
+/* x86: pirq can be used by HVM guests */
+#define XENFEAT_hvm_pirqs           10
+
 #define XENFEAT_NR_SUBMAPS 1
 
 #endif /* __XEN_PUBLIC_FEATURES_H__ */