Allow use of pc machine type (accel=xen) for Xen HVM domains.

Xen HVM domains normally spawn QEMU with a dedicated xenfv machine type. The
initialization code for this machine type can easily be pulled into the
generic pc initialization code and guarded with a test for whether the xen
accelerator options is specified, which is more consistent with the way
other accelerators are used.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
diff --git a/xen-stub.c b/xen-stub.c
index 6f0516a..47c8e73 100644
--- a/xen-stub.c
+++ b/xen-stub.c
@@ -63,3 +63,8 @@
 void xen_modified_memory(ram_addr_t start, ram_addr_t length)
 {
 }
+
+int xen_hvm_init(void)
+{
+    return 0;
+}