vfio/virtio: Use guard() for list_lock where applicable

vfio/virtio: Use guard() for list_lock where applicable

Convert list_lock mutex acquisitions to use guard() and scoped_guard()
where the lock scope aligns with the function or block scope.  This
simplifies virtiovf_get_data_buff_from_pos() by replacing goto-based
unwinding with direct returns.

Assisted-by: Claude:claude-opus-4-6
Signed-off-by: Alex Williamson <alex.williamson@nvidia.com>
Reviewed-by: Yishai Hadas <yishaih@nvidia.com>
Link: https://lore.kernel.org/r/20260414200625.3601509-3-alex.williamson@nvidia.com
Signed-off-by: Alex Williamson <alex@shazbot.org>