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

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

Rewrite lxcDomainAttachDeviceDiskLive 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 4dd3a7d5bc44980135a1b11810ba9aeab42a4a59)

Conflicts:
	src/lxc/lxc_driver.c: OOM + cgroups error reporting and
        remove usernamespace integration