'virsh lxc-enter-namespace' does not have a way to reflect exit
status to the caller in single-command mode, but we might as well
at least report the exit status.  Prior to this patch,

$ virsh -c lxc:/// lxc-enter-namespace shell /bin/sh 'exit 3'; echo $?
1

now it gives some details:

$ virsh -c lxc:/// lxc-enter-namespace shell /bin/sh -c 'exit 3'; echo $?
error: internal error: Child process (31557) unexpected exit status 3
1

Also useful:

$ virsh -c lxc:/// lxc-enter-namespace shell /bin/sh -c 'kill $$'; echo $?
error: internal error: Child process (31585) unexpected fatal signal 15
1

* tools/virsh-domain.c (cmdLxcEnterNamespace): Avoid magic numbers.
Dispatch any error.
* tools/virsh.pod: Document that non-zero exit status is collapsed.
Signed-off-by: NEric Blake <eblake@redhat.com>