exec: make -mem-path filenames deterministic

Adds ramblocks' names to their backing files when using -mem-path.  Eases
introspection and debugging.

Signed-off-by: Peter Feiner <peter@gridcentric.ca>
Message-id: 1362423265-15855-1-git-send-email-peter@gridcentric.ca
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>