drivers: serial: Add UART VIRTIO Console

Add a UART driver for the VIRTIO console device.
This driver has support for both polling- and interrupt-based i/o,
as well as interacting with up to 32 console ports (disabled by
default). Based on the Xen HVC driver.
Tested with the console subsystem sample programs. Aside from
enabling CONFIG_PCIE, setting CONFIG_HEAP_MEM_POOL_SIZE to a
high enough value (for example 100000) is necessary -- as is done
in the virtiofs sample.

Signed-off-by: Jakub Klimczak <jklimczak@internships.antmicro.com>