LXC: Fix virLXCControllerSetupDevPTS() wrt user namespaces

LXC: Fix virLXCControllerSetupDevPTS() wrt user namespaces

The gid value passed to devpts has to be translated by hand as
virLXCControllerSetupDevPTS() is called before setting up the user
and group mappings.
Otherwise devpts will use an unmapped gid and openpty()
will fail within containers.
Linux kernel commit 23adbe12
("fs,userns: Change inode_capable to capable_wrt_inode_uidgid")
uncovered that issue.

Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Ján Tomko <jtomko@redhat.com>