xen: add a dummy xc_hvm_inject_msi for Xen < 4.2

xc_hvm_inject_msi is only available on Xen >= 4.2: add a dummy
compatibility function for Xen < 4.2.

Also enable msi support only on Xen >= 4.2.

Signed-off-by: Stefano Stabellini <[email protected]>
Acked-by: Anthony PERARD <[email protected]>
diff --git a/xen-all.c b/xen-all.c
index a08eec0..bdf9c0f 100644
--- a/xen-all.c
+++ b/xen-all.c
@@ -129,7 +129,7 @@
 
 void xen_hvm_inject_msi(uint64_t addr, uint32_t data)
 {
-    xc_hvm_inject_msi(xen_xc, xen_domid, addr, data);
+    xen_xc_hvm_inject_msi(xen_xc, xen_domid, addr, data);
 }
 
 static void xen_suspend_notifier(Notifier *notifier, void *data)