virt-host-validate: distinguish exists vs accessible for devices

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>