CVE-2013-6456: Avoid unsafe use of /proc/$PID/root in LXC USB hotplug

CVE-2013-6456: Avoid unsafe use of /proc/$PID/root in LXC USB hotplug

Rewrite lxcDomainAttachDeviceHostdevSubsysUSBLive function
to use the virProcessRunInMountNamespace helper. This avoids
risk of a malicious guest replacing /dev with a absolute
symlink, tricking the driver into changing the host OS
filesystem.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
(cherry picked from commit 7fba01c15c1f886b4235825692b4c13e88dd9f7b)

Conflicts:
	src/lxc/lxc_driver.c: OOM + cgroups error reporting