xenapi: ignore 500 errors from agent resetnetwork

Old versions of the FreeBSD and Gentoo agent return 500 on successful
resetnetwork calls. This changes the current error checking to treat 500
as a successful response for resetnetwork calls.

Fixes bug 1223818
Change-Id: Ia3eea109b6c3186af77124c704f13b70b0b91e81
Merge "xenapi: ignore 500 errors from agent resetnetwork"