xenapi: Extract plugin installation functions

This change extracts the plugin installation functions, and covers the
extracted functions with tests. Use:

    ./test_funtions.sh run_tests

to run the tests.

Change-Id: I1d78d9e8cc4d52ee2df83d07e4c74dda4805f21a
Merge "xenapi: Extract plugin installation functions"
Only start zeromq-receiver when we're using zeromq.

Change-Id: Ibd3e6a74f1098e330be0fcbbb83b9188e51202ad