vtpm: Fix hashed-memory file writing.

vtpm: Fix hashed-memory file writing.

There is a bug in the vtpm_manager that has to do with hashing and
saving the NVM memory files (vtpm_dm_%d.data). The file is not
truncated when it is written and this results in the hash becoming
invalid because of the extra bits at the end of the file.

This patch adds O_TRUNC to the flags when opening the file.

More details on this issue are in the bug report on bugzilla=20
http://bugzilla.xensource.com/bugzilla/show_bug.cgi?id=3D1488

Signed-off-by: Matt Fioravante <Matthew.Fioravante@jhuapl.edu>