cli: Add support for virt-viewer

Currently the CLI supports opening a console using the "vncviewer" and
"spicec" commands. This patch adds support for using the "remote-viewer"
command as well. If the "remote-viewer" command is available it will be
preferred.

Change-Id: I036c8f4578c876cf012017b4d3ea19754e41d9c1
Bug-Url: https://bugzilla.redhat.com/807696
Signed-off-by: Juan Hernandez <juan.hernandez@redhat.com>