fix(instances): the quota counts every VM and refuses what it cannot verify

check_user_quota added up the vCPUs, memory and disks of the user's
VMs, but skipped a VM whose host failed a TCP probe. A host that was
down therefore let users exceed their quota, and the probe always failed
for ssh computes named by an ssh_config alias or reached through
ProxyJump, so their VMs were never counted. A VM that could not be read
turned the page into the error page instead.

- Hosts are checked with Compute.status, the connection WebVirtCloud
  uses, once per host. A VM that cannot be read (host down, VM gone)
  makes the usage unknown: clone and resize up are refused with their
  own message (fail closed).
- A request that adds nothing is not checked, so a user over quota can
  still shrink a VM. Users without limits, and an exceeded instance
  count (from the database), need no host either.
- Memory resize of a running VM changes only the current memory, but
  the quota was charged with the submitted maximum. It is now charged
  with no change, and resize_mem gets the current maximum, so a VM that
  shuts down meanwhile cannot take an unchecked one.
- The VM page no longer computes the quota on every load (it read every
  VM of the user on every host for a "quota reached" banner). The limit
  is reported when a change is refused.

The test that expected a down host to pass the quota now expects it to
be refused. Checked against the test hosts: with a VM behind an
ssh_config alias the previous code counted 29 vCPUs, the new one 31;
with a VM on an unreachable host the previous code allowed +1 vCPU,
the new one refuses it after 5.1 s and allows -1 at once. The new
tests fail on the previous code, except the ones guarding unchanged
behaviour.

instances/templates/instance.html

@@ -67,11 +67,6 @@
                 <span>{% bs_icon 'repeat'%} </span>
             </a>
         </div>
-        {% if user_quota_msg %}
-            <div class="alert alert-warning fade show">
-            {{ user_quota_msg|capfirst }} {% trans "quota reached" %}.
-            </div>
-        {% endif %}
         <hr>
     </div>