Push fallocate() down into mkstemp(); use known size

Towards defining the DiskFile class, or something like it, as an API
for the low level disk acesses, we push the fallocate() system call
down into the DiskFile.mkstemp() method. This allows another
implementation of DiskFile to decide to use or not use fallocate().

Change-Id: Ib4d2ee1f971e4e20e53ca4b41892c5e44ecc88d5
Signed-off-by: Peter Portante <peter.portante@redhat.com>
Merge "Push fallocate() down into mkstemp(); use known size"