xenvm: drivers: serial: Add consoleio Xen serial driver for Domain 0

This commit adds Xen consoleio serial driver. It is needed to receive
kernel messages from Zephyr in case it runs as Xen privileged domain
(Dom0). There is no console ring buffer for such domain, so regular
uart_hvc_xen driver can not be used (privileged domain input/output
are possible only through consoleio interface).

Signed-off-by: Dmytro Firsov <dmytro_firsov@epam.com>