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.

computes/test_quota_down_host.py

@@ -6,11 +6,12 @@ from computes.models import Compute
 from django.contrib.auth import get_user_model
 from django.test import TestCase
 from instances.models import Instance
-from instances.utils import check_user_quota
+from instances.utils import QUOTA_UNVERIFIED, check_user_quota
 
 
 class QuotaWithDownHostTestCase(TestCase):
-    def test_vm_on_an_unreachable_host_does_not_break_the_quota_check(self):
+    def test_vm_on_an_unreachable_host_makes_the_quota_unverified(self):
+        # its usage is unknown, so an increase is refused (fail closed), without an error page
         sock = socket.socket()
         sock.bind(("127.0.0.1", 0))
         port = sock.getsockname()[1]
@@ -22,4 +23,4 @@ class QuotaWithDownHostTestCase(TestCase):
         UserInstance.objects.create(user=user, instance=vm)
         get_settings()  # app settings are normally loaded by middleware
 
-        self.assertEqual(check_user_quota(user, 1, 1, 128, 1), "")
+        self.assertEqual(check_user_quota(user, 1, 1, 128, 1), QUOTA_UNVERIFIED)