trace: don't abort qemu if ftrace can't be initialized

trace: don't abort qemu if ftrace can't be initialized

If the ftrace backend is compiled into QEMU, any attempt
to start QEMU while non-root will fail due to the
inability to open /sys/kernel/debug/tracing/tracing_on.

Add a fallback into the code so that it connects up the
trace_marker_fd variable to /dev/null when getting
EACCES on the 'trace_on' file. This allows QEMU to
run, with ftrace turned into a no-op.

[Fixed s/setting/getting/ and s/EACCESS/EACCES/ errors pointed out by
Eric Blake <[email protected]>.
--Stefan]

Signed-off-by: Daniel P. Berrange <[email protected]>
Message-id: 1475588159[email protected]
Signed-off-by: Stefan Hajnoczi <[email protected]>