perf trace: Fix comm resolution when reading events from file

Task comm's are getting lost when processing events from a file. The problem is
that the trace struct has its host machine and perf-session has its host
machine. When processing task events from a file the perf-session machine is
used, but when processing sample events the trace host machine is used. Fix by
having both references point to the same machine.

Before:

     0.030 ( 0.001 ms): :27743/27743 brk( ...
     0.057 ( 0.004 ms): :27743/27743 mmap(len: 4096, prot: READ|WRITE, flags: ...
     0.075 ( 0.006 ms): :27743/27743 access(filename: 0x7f3809fbce00, mode: R ...
     0.091 ( 0.005 ms): :27743/27743 open(filename: 0x7f3809fba14c, flags: CLOEXEC ...
...

After:
     0.030 ( 0.001 ms): make/27743 brk( ...
     0.057 ( 0.004 ms): make/27743 mmap(len: 4096, prot: READ|WRITE, flags: ...
     0.075 ( 0.006 ms): make/27743 access(filename: 0x7f3809fbce00, mode: R ...
     0.091 ( 0.005 ms): make/27743 open(filename: 0x7f3809fba14c, flags: CLOEXEC ...
...

Signed-off-by: David Ahern <dsahern@gmail.com>
perf trace: Fix summary when processing events from a file

Needs to be folded into summary patch

Signed-off-by: David Ahern <dsahern@gmail.com>