virt-host-validate: distinguish exists vs accessible for devices
    
    Currently we just check that various devices are accessible.
    This leads to inaccurate errors reported for /dev/kvm and
    /dev/vhost-net if they exist but an unprivileged user lacks
    access. Switch existing checks to look for file existance,
    and add a separate check for accessibility of /dev/kvm
    since some distros don't grant users access by default.
    
    Signed-off-by: Daniel P. Berrange <berrange@redhat.com>