trace: Add ftrace tracing backend

trace: Add ftrace tracing backend

This patch adds a ftrace tracing backend which sends trace event to
ftrace marker file. You can effectively compare qemu trace data and
kernel(especially, kvm.ko when using KVM) trace data.
The ftrace backend is restricted to Linux only.

To try out the ftrace backend:

 $ ./configure --trace-backend=ftrace
 $ make

if you use KVM, enable kvm events in ftrace:

 # sudo echo 1 > /sys/kernel/debug/tracing/events/kvm/enable

After running qemu by root user, you can get the trace:

 # cat /sys/kernel/debug/tracing/trace

Signed-off-by: Eiichi Tsukata <eiichi.tsukata.xh@hitachi.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>