trace: move qemu_trace_opts to trace/control.c

trace: move qemu_trace_opts to trace/control.c

The patch also creates trace_opt_parse() helper in trace/control.c to reuse
this code in next patches for qemu-nbd and qemu-io.

The patch also makes trace_init_events() static, as this call is not used
outside the module anymore.

Signed-off-by: Denis V. Lunev <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Reviewed-by: Stefan Hajnoczi <[email protected]>
Message-id: 1466174654[email protected]
CC: Paolo Bonzini <[email protected]>
CC: Stefan Hajnoczi <[email protected]>
CC: Kevin Wolf <[email protected]>
Signed-off-by: Stefan Hajnoczi <[email protected]>