util: Check for pkttyagent availability properly
util: Check for pkttyagent availability properly

It does not need a tty to work, it opens its controlling terminal for user
interaction and with this patch even crazy things like this work:

  echo 'list --name' | virsh -q >/dev/null

Signed-off-by: Martin Kletzander <e82f23e74d43760279ebcc505dac4ae70d5627d0@redhat.com>
Reviewed-by: Ján Tomko <4cab11cfb98d3c937327354a78eb07dbb6ee2bc6@redhat.com>

jfehlig/libvirt,jfehlig/libvirt,libvirt/libvirt,olafhering/libvirt,libvirt/libvirt,libvirt/libvirt,crobinso/libvirt,zippy2/libvirt,crobinso/libvirt,olafhering/libvirt,zippy2/libvirt,crobinso/libvirt,crobinso/libvirt,zippy2/libvirt,zippy2/libvirt,olafhering/libvirt,jfehlig/libvirt,jfehlig/libvirt,olafhering/libvirt,libvirt/libvirt