MSI-X vector Mask Bits are in MSI-X Tables in PCI memory space.  Xen PV
guests can't write to those tables.  MSI vector Mask Bits are in PCI
configuration space.  Xen PV guests can write to config space, but those
writes are ignored.

Commit 0e4ccb15 ("PCI: Add x86_msi.msi_mask_irq() and
msix_mask_irq()") added a way to override default_mask_msi_irqs() and
default_mask_msix_irqs() so they can be no-ops in Xen guests, but this is
more complicated than necessary.

Add "pci_msi_ignore_mask" in the core PCI MSI code.  If set,
default_mask_msi_irqs() and default_mask_msix_irqs() return without doing
anything.  This is less flexible, but much simpler.

[bhelgaas: changelog]
Signed-off-by: NYijing Wang <wangyijing@huawei.com>
Signed-off-by: NBjorn Helgaas <bhelgaas@google.com>
Reviewed-by: NDavid Vrabel <david.vrabel@citrix.com>
CC: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
CC: xen-devel@lists.xenproject.org