qemu: Introduce 16550A serial console model

qemu: Introduce 16550A serial console model

None of the existing models is suitable for use with
RISC-V virt guests, and we don't want information about
the serial console to be missing from the XML.

The name is based on comments in qemu/hw/riscv/virt.c:

  RISC-V machine with 16550a UART and VirtIO MMIO

and in qemu/hw/char/serial.c:

  QEMU 16550A UART emulation

along with the output of dmesg in the guest:

  Serial: 8250/16550 driver, 4 ports, IRQ sharing disabled
  10000000.uart: ttyS0 at MMIO 0x10000000 (irq = 13,
    base_baud= 230400) is a 16550A

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>