vfio-user: disconnect container when device info query fails

vfio_user_device_attach() connects the vfio-user container before querying
VFIO_USER_DEVICE_GET_INFO.  If the device info query fails,
vfio_device_prepare() has not run yet, so vbasedev->bcontainer is still
NULL and the later vfio_device_detach() cleanup path cannot reach the new
container.

Disconnect the container before returning the attach failure so the listener,
RAM discard state, object reference and address space reference are released
on this error path.

Signed-off-by: GuoHan Zhao <zhaoguohan@kylinos.cn>
Reviewed-by: John Levon <john.levon@nutanix.com>
Link: https://lore.kernel.org/qemu-devel/20260522065637.4109499-1-zhaoguohan@kylinos.cn
Signed-off-by: Cédric Le Goater <clg@redhat.com>