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>

docs/formatdomain.html.in

@@ -7011,7 +7011,8 @@ qemu-kvm -net nic,model=? /dev/null
       is available) and <code>pci-serial</code> (usable whenever PCI support
       is available); <span class="since">since 3.10.0</span>,
       <code>spapr-vio-serial</code> (usable with ppc64/pseries guests),
-      <code>system-serial</code> (usable with aarch64/virt guests) and
+      <code>system-serial</code> (usable with aarch64/virt and,
+      <span class="since">since 4.7.0</span>, riscv/virt guests) and
       <code>sclp-serial</code> (usable with s390 and s390x guests) are
       available as well.
     </p>
@@ -7025,10 +7026,11 @@ qemu-kvm -net nic,model=? /dev/null
       target type); <code>pci-serial</code>
       (usable with the <code>pci-serial</code> target type);
       <code>spapr-vty</code> (usable with the <code>spapr-vio-serial</code>
-      target type); <code>pl011</code> (usable with the
-      <code>system-serial</code> target type); <code>sclpconsole</code> and
-      <code>sclplmconsole</code> (usable with the <code>sclp-serial</code>
-      target type).
+      target type); <code>pl011</code> and,
+      <span class="since">since 4.7.0</span>, <code>16550a</code> (usable
+      with the <code>system-serial</code> target type);
+      <code>sclpconsole</code> and <code>sclplmconsole</code> (usable with
+      the <code>sclp-serial</code> target type).
     </p>
 
     <p>

docs/schemas/domaincommon.rng

@@ -3733,6 +3733,7 @@
           <value>pci-serial</value>
           <value>spapr-vty</value>
           <value>pl011</value>
+          <value>16550a</value>
           <value>sclpconsole</value>
           <value>sclplmconsole</value>
         </choice>