xenapi: ensure agent check respects image flags

The tests assumed sys_meta was in a simpler format
when accessed on the compute node. This change fixes
the code to work with the actual format of sys_meta.

Fixes bug 1190198
Change-Id: I31095e25bc86c8a475a58a7325e1ff2b3ae8e15c
Merge "xenapi: ensure agent check respects image flags"