Duration analysis is based on trace pairs which follow the convention
in which function entry generates trace named X and ends with either
trace X_ret or X_err. Traces which do not have an accompanying return
tracepoint are ignored.

New commands:

  osv trace summary

      Prints execution time statistics for traces

  osv trace duration {function}

      Prints timed traces sorted by duration in descending order.
      Optionally narrowed down to a specified function

gdb$ osv trace summary
Execution times [ms]:
name          count      min      50%      90%      99%    99.9%      max    total
vfs_pwritev       3    0.682    1.042    1.078    1.078    1.078    1.078    2.801
vfs_pwrite       32    0.006    1.986    3.313    6.816    6.816    6.816   53.007

gdb$ osv trace duration
0xffffc000671f0010  1    1385318632.103374   6.816 vfs_pwrite
0xffffc0003bbef010  0    1385318637.929424   3.923 vfs_pwrite

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>