vpci/msi: fix unbind loop

The current unbind loop on failure in vpci_msi_enable is wrong and
will only work correctly if the initial pirq is 0. Fix this by adding
a proper bound.

Reported-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Release-acked-by: Juergen Gross <jgross@suse.com>

xen/arch/x86/hvm/vmsi.c

@@ -710,7 +710,7 @@ static int vpci_msi_enable(const struct pci_dev *pdev, uint32_t data,
                      "%04x:%02x:%02x.%u: failed to bind PIRQ %u: %d\n",
                      pdev->seg, pdev->bus, PCI_SLOT(pdev->devfn),
                      PCI_FUNC(pdev->devfn), pirq + i, rc);
-            while ( bind.machine_irq-- )
+            while ( bind.machine_irq-- > pirq )
                 pt_irq_destroy_bind(pdev->domain, &bind);
             spin_lock(&pdev->domain->event_lock);
             unmap_domain_pirq(pdev->domain, pirq);