xen/pvh: don't try to unplug emulated devices

A Xen PVH guest has no associated qemu device model, so trying to
unplug any emulated devices is making no sense at all.

Bail out early from xen_unplug_emulated_devices() when running as PVH
guest. This will avoid issuing the boot message:

[    0.000000] Xen Platform PCI: unrecognised magic value

Cc: <stable@vger.kernel.org> # 4.11
Signed-off-by: NJuergen Gross <jgross@suse.com>
Reviewed-by: NBoris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: NJuergen Gross <jgross@suse.com>

arch/x86/xen/platform-pci-unplug.c

@@ -134,6 +134,10 @@ void xen_unplug_emulated_devices(void)
 {
 	int r;
 
+	/* PVH guests don't have emulated devices. */
+	if (xen_pvh_domain())
+		return;
+
 	/* user explicitly requested no unplug */
 	if (xen_emul_unplug & XEN_UNPLUG_NEVER)
 		return;