Update 'list_users' call to use AGENT_HIGH_TIMEOUT

The list_users call in the guestagent takes about 0.4s - 0.5s per
user, and currently uses the AGENT_LOW_TIMEOUT. This means that it's
possible for the call to easily time out if there are ~11-12 users
on the guest.

This has been causing a lot of false negative failures in the
int-tests, so this updates the 'list_users' call to use the
AGENT_HIGH_TIMEOUT instead of the AGENT_LOW_TIMEOUT.

Closes-bug: 1356085
Change-Id: I589baa0e95febfb63538b79a082cc3dc55f66510