xenapi plugins: Make sure subprocesses finish executing

Fixes bug 1286187

Neither execute_get_output() nor execute() would wait for the process to
fully finish executing. This could potentially create a race condition
where commands execute in a different order than intended.

Also, for execute() it could cause a process to never finish executing if
it generates enough output to block writing to the pipe.

Change-Id: I3404f4b3ca1cddeec2f3e7b393817a6ccc42bec7
Merge "xenapi plugins: Make sure subprocesses finish executing"