If one pastes from the output of virt-sansbox-image

  $ virt-sandbox-image list
  docker:/library/ubuntu?tag=17.04
  docker:/library/debian?tag=latest

verbatim

  $ virt-sandbox-image run -c qemu:///session

 docker:/library/debian?tag=latest

This fails like

  Unable to start sandbox: Failed to create domain: XML error: name library/debian:qbeilwxard cannot contain '/'

so don't allow invalid chars like '/' in domain names

Signed-off-by: Guido Günther <agx@sigxcpu.org>
Reviewed-by: Daniel P. Berrange <berrange@redhat.com>