health-server: Improving error message for network tests

If the ip-based filtering (by using network-hosts) was not matching
enough VMs, the resulting message was about some affinity issues ...
which is not the right error here.

This patch is so improving as follow :
- if the number of hosts _before_ doing the ip-filtering is less than the
  expected number, there is no need to continue. Let's inform the user we don't have enought VMs.

- if the number of hosts _after_ the ip-filtering is less than the
  expected number, there is no need to continue. Let's inform the user
  that we don't have enought VMs after the ip filtering.

This way users can know why we don't have enough VMs for this test.