xen/pvh: Setup gsi for passthrough device

In PVH dom0, the gsis don't get registered, but the gsi of
a passthrough device must be configured for it to be able to be
mapped into a domU.

When assigning a device to passthrough, proactively setup the gsi
of the device during that process.

Signed-off-by: Jiqian Chen <Jiqian.Chen@amd.com>
Signed-off-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Jiqian Chen <Jiqian.Chen@amd.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Message-ID: <20240924061437.2636766-3-Jiqian.Chen@amd.com>
Signed-off-by: Juergen Gross <jgross@suse.com>