util: simplify virCommand APIs for env passthrough.

util: simplify virCommand APIs for env passthrough.

Now that 100% of libvirt code is forbidden in a SUID environment,
we no longer need to worry about whether env variables are
trustworthy or not. The virt-login-shell setuid program, which
does not link to any libvirt code, will purge all environment
variables, except $TERM, before invoking the virt-login-shell-helper
program which uses libvirt.

Thus we only need one API for env passthrough in virCommand.

Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>