vfio: Adapt drivers to use the core helper vfio_check_precopy_ioctl

vfio: Adapt drivers to use the core helper vfio_check_precopy_ioctl

Introduce a core helper function for VFIO_MIG_GET_PRECOPY_INFO and adapt
all drivers to use it.

It centralizes the common code and ensures that output flags are cleared
on entry, in case user opts in to VFIO_DEVICE_FEATURE_MIG_PRECOPY_INFOv2.
This preventing any unintended echoing of userspace data back to
userspace.

Signed-off-by: Yishai Hadas <yishaih@nvidia.com>
Link: https://lore.kernel.org/r/20260317161753.18964-4-yishaih@nvidia.com
Signed-off-by: Alex Williamson <alex@shazbot.org>