x86/MSI-X: correctly track interrupt masking state

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@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Release-acked-by: Wei Liu <wei.liu2@citrix.com>