xenvm: drivers: serial: add interrupt-driven API for Xen PV console

This commit adds support of interrupt-driven API for UART-like Xen PV
console driver. It is implemented via Xen event channels. It allows to
send and receive data by chunks (not single symbols) and without
polling.

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