The virDomainLxcEnterNamespace method mistakenly uses
virCheckFlags, which returns immediately instead of
virCheckFlagsGoto which jumps to the error cleanup
patch where there is a virDispatchError call

Add a new virDomainLxcEnterSecurityLabel() function as a
counterpart to virDomainLxcEnterNamespaces(), which can
change the current calling process to have a new security
context. This call runs client side, not in libvirtd
so we can't use the security driver infrastructure.

When entering a namespace, the process spawned from virsh
will default to running with the security label of virsh.
The actual desired behaviour is to run with the security
label of the container most of the time. So this changes
virsh lxc-enter-namespace command to invoke the
virDomainLxcEnterSecurityLabel method.

The current behaviour is:

LABEL                             PID TTY          TIME CMD
system_u:system_r:svirt_lxc_net_t:s0:c0.c1023 1 pts/0 00:00:00 systemd
system_u:system_r:svirt_lxc_net_t:s0:c0.c1023 3 pts/1 00:00:00 sh
system_u:system_r:svirt_lxc_net_t:s0:c0.c1023 24 ? 00:00:00 systemd-journal
system_u:system_r:svirt_lxc_net_t:s0:c0.c1023 29 ? 00:00:00 dhclient
staff_u:unconfined_r:unconfined_t:s0-s0:c0.c1023 47 ? 00:00:00 ps

Note the ps command is running as unconfined_t,  After this patch,

The new behaviour is this:

virsh -c lxc:/// lxc-enter-namespace dan -- /bin/ps -eZ
LABEL                             PID TTY          TIME CMD
system_u:system_r:svirt_lxc_net_t:s0:c0.c1023 1 pts/0 00:00:00 systemd
system_u:system_r:svirt_lxc_net_t:s0:c0.c1023 3 pts/1 00:00:00 sh
system_u:system_r:svirt_lxc_net_t:s0:c0.c1023 24 ? 00:00:00 systemd-journal
system_u:system_r:svirt_lxc_net_t:s0:c0.c1023 32 ? 00:00:00 dhclient
system_u:system_r:svirt_lxc_net_t:s0:c0.c1023 38 ? 00:00:00 ps

The '--noseclabel' flag can be used to skip security labelling.
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>

This patch introduces support for LXC specific public APIs. In
common with what was done for QEMU, this creates a libvirt_lxc.so
library and libvirt/libvirt-lxc.h header file.

The actual APIs are

  int virDomainLxcOpenNamespace(virDomainPtr domain,
                                int **fdlist,
                                unsigned int flags);

  int virDomainLxcEnterNamespace(virDomainPtr domain,
                                 unsigned int nfdlist,
                                 int *fdlist,
                                 unsigned int *noldfdlist,
                                 int **oldfdlist,
                                 unsigned int flags);

which provide a way to use the setns() system call to move the
calling process into the container's namespace. It is not
practical to write in a generically applicable manner. The
nearest that we could get to such an API would be an API which
allows to pass a command + argv to be executed inside a
container. Even if we had such a generic API, this LXC specific
API is still useful, because it allows the caller to maintain
the current process context, in particular any I/O streams they
have open.

NB the virDomainLxcEnterNamespace() API is special in that it
runs client side, so does not involve the internal driver API.
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>