hvc_xen: implement multiconsole support

This patch implements support for multiple consoles:
consoles other than the first one are setup using the traditional xenbus
and grant-table based mechanism.
We use a list to keep track of the allocated consoles, we don't
expect too many of them anyway.

Changes in v3:

- call hvc_remove before removing the console from xenconsoles;
- do not lock xencons_lock twice in the destruction path;
- use the DEFINE_XENBUS_DRIVER macro.

Signed-off-by: Stefano Stabellini <[email protected]>
Signed-off-by: Konrad Rzeszutek Wilk <[email protected]>

Author: Stefano Stabellini