x86/MSI: fail if no hardware support

This is to guard against buggy callers (luckily Dom0 only) invoking
the respective hypercall for a device not being MSI-capable.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
While for a well behaved Dom0 this is a benign change, I still think
this should go into 4.6.

--- a/xen/arch/x86/msi.c
+++ b/xen/arch/x86/msi.c
@@ -692,6 +692,8 @@ static int msi_capability_init(struct pc
 
     ASSERT(spin_is_locked(&pcidevs_lock));
     pos = pci_find_cap_offset(seg, bus, slot, func, PCI_CAP_ID_MSI);
+    if ( !pos )
+        return -ENODEV;
     control = pci_conf_read16(seg, bus, slot, func, msi_control_reg(pos));
     maxvec = multi_msi_capable(control);
     if ( nvec > maxvec )