simpletrace: Thread-safe tracing

Trace events outside the global mutex cannot be used with the simple
trace backend since it is not thread-safe.  There is no check to prevent
them being enabled so people sometimes learn this the hard way.

This patch restructures the simple trace backend with a ring buffer
suitable for multiple concurrent writers.  A writeout thread empties the
trace buffer when threshold fill levels are reached.  Should the
writeout thread be unable to keep up with trace generation, records will
simply be dropped.

Each time events are dropped a special record is written to the trace
file indicating how many events were dropped.  The event ID is
0xfffffffffffffffe and its signature is dropped(uint32_t count).

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