vfio/virtio: Use guard() for migf->lock where applicable

vfio/virtio: Use guard() for migf->lock where applicable

Convert migf->lock acquisitions in virtiovf_disable_fd() and
virtiovf_save_read() to use guard().  In virtiovf_save_read() this
eliminates the out_unlock label and multiple goto paths by allowing
direct returns, and removes the need for the done variable to double
as an error carrier.

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/20260414200625.3601509-4-alex.williamson@nvidia.com
Signed-off-by: Alex Williamson <alex@shazbot.org>