xen_console: support the new extended xenstore protocol

Since CS 21994 on xen-unstable.hg and CS
466608f3a32e1f9808acdf832a5843af37e5fcec on qemu-xen-unstable.git, few
changes have been introduced to the PV console xenstore protocol, as
described by the document docs/misc/console.txt under xen-unstable.hg.

From the Qemu point of view, very few modifications are needed to
correctly support the protocol: read from xenstore the "output" node
that tell us what the output of the PV console is going to be.
In case the output is a tty, write to xenstore the device name.

Changes in v2:

- fix error paths: free malloc'ed strings and close the xenstore
connection before returning;

- remove useless snprintf in xenstore_store_pv_console_info if i == 0.

Changes in v3:

- replace xs_daemon_open/xs_daemon_close with xs_open/xs_close.

Changes in v4:

- add a compatibility implementation of xs_open/xs_close.

Changes in v5:

- fix code style.

[agraf] fix build error due to missing stub

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Alexander Graf <agraf@suse.de>
