vfio/container: ram discard disable helper

vfio/container: ram discard disable helper

Define a helper to set ram discard disable, generate error messages,
and cleanup on failure.  The second vfio_ram_block_discard_disable
call site now performs VFIO_GROUP_UNSET_CONTAINER immediately on failure,
instead of relying on the close of the container fd to do so in the kernel,
but this is equivalent.

Signed-off-by: Steve Sistare <steven.sistare@oracle.com>
Reviewed-by: Cedric Le Goater <clg@redhat.com>
Link: https://lore.kernel.org/qemu-devel/1746195760-101443-2-git-send-email-steven.sistare@oracle.com
[ clg: vfio_attach_discard_disable() -> vfio_container_attach_discard_disable() ]
Signed-off-by: Cédric Le Goater <clg@redhat.com>