Improve speed of Xen hypervisor unit tests

Improve speed of Xen hypervisor unit tests

Due to hard-coded timeouts used when listing instances, the Xen
unit tests took around 60s to execute. The key offender was a timeout
of five seconds used for an unsuccessful listing of instances. This
patch refactors the code slightly to define the timeout and delays used
in one place, allowing these to be changed during testing to a more
acceptable value. As a result, these tests take around 5s to execute.

Signed-off-by: Hrvoje Ribicic <riba@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>