rpc: fix escaping of shell path for netcat binary

Consider having a nc binary in the path with a space in its name,
for example '/tmp/fo o/nc'

This results in libvirt running SSH with the following arg value

"'if ''/tmp/fo o/nc'' -q 2>&1 | grep \"requires
an argument\" >/dev/null 2>&1; then ARG=-q0;
else ARG=;fi;''/tmp/fo o/nc'' $ARG -U
/var/run/libvirt/libvirt-sock'"

The use of the single quote escaping was introduced by

commit 6ac6238de33fc74e7545b245ae273d1bfd658808
Author: Guido Günther <agx@sigxcpu.org>
Date: Thu Oct 13 21:49:01 2011 +0200

Use virBufferEscapeShell in virNetSocketNewConnectSSH

to escape the netcat command since it's passed to the shell. Adjust
expected test case output accordingly.

While the intention of this change was good, the result is broken as it
is still underquoted.

On the SSH server side, SSH itself runs the command via the shell.
Our command is then invoking the shell again. Thus we see

$ virsh -c qemu+ssh://root@domokun/system?netcat=%2Ftmp%2Ffo%20o%2Fnc list
error: failed to connect to the hypervisor
error: End of file while reading data: sh: /tmp/fo: No such file or directory: Input/output error

With the second level of escaping added we can now successfully use a nc
binary with a space in the path.

The original test case added was misleading as it illustrated using a
binary path of 'nc -4' which is not a path, it is a command with a
separate argument, which is getting interpreted as a path.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>