Our QA team wants to preserve serial output of the guest in between QEMU
runs to perform post-analysis.

By default this behavior is off (file is truncated each time QEMU is
started or device is plugged).

Signed-off-by: Olga Krishtal <okrishtal@virtuozzo.com>
Signed-off-by: Denis V. Lunev <den@openvz.org>
CC: Eric Blake <eblake@redhat.com>
CC: Markus Armbruster <armbru@redhat.com>
CC: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <1449211324-17856-1-git-send-email-den@openvz.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>