x86/pci/xen: Set MSI_FLAG_PCI_MSIX support in Xen MSI domain

The Xen MSI → PIRQ magic does support MSI-X, so advertise it.

(In fact it's better off with MSI-X than MSI, because it's actually
broken by design for 32-bit MSI, since it puts the high bits of the
PIRQ# into the high 32 bits of the MSI message address, instead of the
Extended Destination ID field which is in bits 4-11.

Strictly speaking, this really fixes a much older commit 2e4386eb
("x86/xen: Wrap XEN MSI management into irqdomain") which failed to set
the flag. But that never really mattered until __pci_enable_msix_range()
started to check and bail out early. So in 6.2-rc we see failures e.g.
to bring up networking on an Amazon EC2 m4.16xlarge instance:

[   41.498694] ena 0000:00:03.0 (unnamed net_device) (uninitialized): Failed to enable MSI-X. irq_cnt -524
[   41.498705] ena 0000:00:03.0: Can not reserve msix vectors
[   41.498712] ena 0000:00:03.0: Failed to enable and set the admin interrupts

Side note: This is the first bug found, and first patch tested, by running
Xen guests under QEMU/KVM instead of running under actual Xen.

Fixes: 99f3d279

 ("PCI/MSI: Reject MSI-X early")
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/4bffa69a949bfdc92c4a18e5a1c3cbb3b94a0d32.camel@infradead.org