qemu: Process new 'append' attribute for char dev with output to a file

qemu: Process new 'append' attribute for char dev with output to a file

By default, QEMU truncates serial file on open. Sometimes, it could be weird -
for example, when we are trying to investigate some event, which occured several
restarts ago. This patch adds an ability to preserve previous content.

Signed-off-by: Dmitry Mishin <dim@virtuozzo.com>