sync: Flush all caches when destroying the pod

[sboeuf]

sync: Flush all caches when destroying the pod

When DESTROYPOD command is called, this means the VM is going to be
shut down right after. Because we don't want to shut down the VM with
some pending buffers, this patch calls into sync() system call to make
sure all buffers are flushed.

From sync() man page:
"According to the standard specification (e.g., POSIX.1-2001), sync()
 schedules the writes, but may return before the actual writing is
 done. However Linux waits for I/O completions, and thus sync() or
 syncfs() provide the same guarantees as fsync called on every file
 in the system or filesystem respectively."

This will ensure that sync() will block until all buffers are properly
flushed.

Fixes #168

Signed-off-by: Sebastien Boeuf <sebastien.boeuf@intel.com>

[sboeuf]

cc @mcastelino @amshinde @sameo

[mcastelino]

LGTM

[sameo]

LGTM