conf: use disk source accessors in lxc/

conf: use disk source accessors in lxc/

Part of a series of cleanups to use new accessor methods.

* src/lxc/lxc_cgroup.c (virLXCCgroupSetupDeviceACL): Use
accessors.
* src/lxc/lxc_controller.c (virLXCControllerSetupLoopDeviceDisk)
(virLXCControllerSetupNBDDeviceDisk)
(virLXCControllerSetupLoopDevices, virLXCControllerSetupDisk):
Likewise.
* src/lxc/lxc_driver.c (lxcDomainAttachDeviceDiskLive)
(lxcDomainDetachDeviceDiskLive): Likewise.

Signed-off-by: Eric Blake <eblake@redhat.com>