xen/arm: vpl011: Add support for vuart in libxl

xen/arm: vpl011: Add support for vuart in libxl

An option is provided in libxl to enable/disable SBSA vuart while
creating a guest domain.

Libxl now supports a generic vuart console and SBSA uart is a specific type.
In future support can be added for multiple vuart of different types.

User can enable SBSA vuart by adding the following line in the guest
configuration file:

vuart = "sbsa_uart"

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