xen/arm: vpl011: Modify xenconsole to define and use a new console structure

xen/arm: vpl011: Modify xenconsole to define and use a new console structure

Xenconsole uses a domain structure which contains console specific fields. This
patch defines a new console structure, which would be used by the xenconsole
functions to perform console specific operations like reading/writing data from/to
the console ring buffer or reading/writing data from/to console tty.

This patch is in preparation to support multiple consoles to support vuart console.

Signed-off-by: Bhupinder Thakur <bhupinder.thakur@linaro.org>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Acked-by: Wei Liu <wei.liu2@citrix.com>