[libvirt] Add new API for accessing remote guest text console

[Daniel P. Berrange]

The 'virsh console' command has been an oddity that only works
when run locally, as the same UID as the QEMU instance. This
is because it directly opens /dev/pty/XXX. This introduces a
formal API for accessing consoles that uses the virStreamPtr
APIs. Now any app can open consoles anywhere it can connect
to libvirt