trace: Add trace-file command to open/close/flush trace file

trace: Add trace-file command to open/close/flush trace file

This patch adds the trace-file command:

  trace-file [on|off|flush]

  Open, close, or flush the trace file.  If no argument is given,
  the status of the trace file is displayed.

The trace file is turned on by default but is only written out when the
trace buffer becomes full.  The flush operation can be used to force
write out at any time.

Turning off the trace file does not change the state of trace events;
tracing will continue to the trace buffer.  When the trace file is off,
use "info trace" to display the contents of the trace buffer in memory.

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
This commit also contains the trace-file sub-command from the following
commit:

commit 5ce8d1a957afae2c52ad748944ce72848ccf57bd
Author: Prerna Saxena <prerna@linux.vnet.ibm.com>
Date:   Wed Aug 4 16:23:54 2010 +0530

    trace: Add options to specify trace file name at startup and runtime

    This patch adds an optional command line switch '-trace' to specify the
    filename to write traces to, when qemu starts.
    Eg, If compiled with the 'simple' trace backend,
    [temp@system]$ qemu -trace FILENAME IMAGE
    Allows the binary traces to be written to FILENAME instead of the option
    set at config-time.

    Also, this adds monitor sub-command 'set' to trace-file commands to
    dynamically change trace log file at runtime.
    Eg,
    (qemu)trace-file set FILENAME
    This allows one to set trace outputs to FILENAME from the default
    specified at startup.

    Signed-off-by: Prerna Saxena <prerna@linux.vnet.ibm.com>
    Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>