qemu: command: support -chardev for platform devices

qemu: command: support -chardev for platform devices

Some qemu arch/machine types have built in platform devices that
are always implicitly available. For platform serial devices, the
current code assumes that only old style -serial config can be
used for these devices.

Apparently though since -chardev was introduced, we can use -chardev
in these cases, like this:

  -chardev pty,id=foo
  -serial chardev:foo

Since -chardev enables all sorts of modern features, use this method
for platform devices.

Reviewed-by: Andrea Bolognani <abologna@redhat.com>
Signed-off-by: Cole Robinson <crobinso@redhat.com>