xen/arm: vpl011: Add support for multiple consoles in xenconsole

xen/arm: vpl011: Add support for multiple consoles in xenconsole

This patch adds the support for multiple consoles and introduces the
iterator functions to operate on multiple consoles.

The functions called by the iterators check that they are operating
on valid I/O parameters. This ensures that if a particular console is
not initialized then the functions will not do anything for that
console type.

This patch is in preparation to support a new vuart console.

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