vfio/spapr: Enhance error handling in vfio_spapr_create_window()

vfio/spapr: Enhance error handling in vfio_spapr_create_window()

Introduce an Error ** parameter to vfio_spapr_create_window() to enable
structured error reporting. This allows the function to propagate
detailed errors back to callers.

Suggested-by: Cédric Le Goater <clg@redhat.com>
Signed-off-by: Amit Machhiwal <amachhiw@linux.ibm.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Link: https://lore.kernel.org/qemu-devel/20250408124042.2695955-2-amachhiw@linux.ibm.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>