qemu: Support chardevs with ARM virt machines

qemu: Support chardevs with ARM virt machines

The function to check if -chardev is supported by QEMU was written a
long time ago, where adding chardevs did not make sense on the fixed ARM
platforms.  Since then, we now have a general purpose virt platform,
which should support plugging in any device over PCIe which is supported
in a similar fashion on x86.

Signed-off-by: Christoffer Dall <cdall@linaro.org>
Reviewed-by: Andrea Bolognani <abologna@redhat.com>