x86/MSI-X: correctly track interrupt masking state

When a guest unmasks MSI-X interrupts before enabling MSI-X on the
device, so far nothing updates the {host,guest}_masked internal state;
this to date only gets done when MSI-X is already enabled. This is why
half way recent Linux works (as it enables MSI-X first), while Windows
doesn't (as it enables MSI-X only after having set up und unmasked all
vectors). Since with a successful write to the vector control field
everything is ready internally, we should also update internal tracking
state there, regardless of the device's MSI-X enabled state.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/msi.c
+++ b/xen/arch/x86/msi.c
@@ -434,8 +434,13 @@ static bool_t msi_set_mask_bit(struct ir
         {
             writel(flag, entry->mask_base + PCI_MSIX_ENTRY_VECTOR_CTRL_OFFSET);
             readl(entry->mask_base + PCI_MSIX_ENTRY_VECTOR_CTRL_OFFSET);
+
             if ( likely(control & PCI_MSIX_FLAGS_ENABLE) )
                 break;
+
+            entry->msi_attrib.host_masked = host;
+            entry->msi_attrib.guest_masked = guest;
+
             flag = 1;
         }
         else if ( flag && !(control & PCI_MSIX_FLAGS_MASKALL) )