x86/pt: add a MSI unmask flag to XEN_DOMCTL_bind_pt_irq

x86/pt: add a MSI unmask flag to XEN_DOMCTL_bind_pt_irq

The flag is part of the gflags, and should be used to request the
unmask of a MSI interrupt once it's bound.

This is required for the device model in order to be capable of
binding MSIX interrupts that have the entry mask bit already unset at
bind time. Without this fix the interrupts would be left masked.

Note that this commit introduces a change to the domctl, which
requires a bump of the interface version. This is not done here
because the interface version has already been bumped in this release
cycle.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reported by: Andreas Kinzler <hfp@posteo.de>
Reviewed-by: Jan Beulich <jbeulich@suse.com>