refactor(computes): the Computes page reads capacity through vrtManager

The capacity read lived in its own module with raw libvirt calls and a
thread pool with a deadline, beside helpers that already read the same
things. It now uses them:

- wvmHostDetails.get_capacity() sums the vCPUs and memory of all and of
  the running VMs from get_host_instances(), against the host's info.
- get_storages_info() also gives each pool's free space.
- Compute.capacity joins the two, like cpu_count and ram_size, and is
  None when the host is down or the read fails.

The page reads the hosts one after another, as it already connected to
each of them; a host that cannot be reached is given up after the
connect timeout. computes/capacity.py and its thread handling are gone.
The page looks and sorts as before.

computes/models.py

@@ -4,11 +4,12 @@ from django.db import transaction
 from django.db.models import CharField, IntegerField, Model
 from django.utils.functional import cached_property
 from django.utils.translation import gettext_lazy as _
-from libvirt import virConnect
+from libvirt import libvirtError, virConnect
 
 from computes.validators import validate_hostname, validate_login, validate_migration_address
 from vrtManager.connection import connection_manager
 from vrtManager.hostdetails import wvmHostDetails
+from vrtManager.storage import wvmStorages
 
 
 class Compute(Model):
@@ -87,6 +88,20 @@ class Compute(Model):
     def ram_usage(self):
         return self.proxy.get_memory_usage()["percent"]
 
+    @cached_property
+    def capacity(self):
+        """What the host has given its VMs (wvmHostDetails.get_capacity) and
+        its active storage pools, or None when it cannot be read."""
+        if self.status is not True:
+            return None
+        try:
+            given = self.proxy.get_capacity()
+            storages = wvmStorages(self.hostname, self.login, self.password, self.type)
+            given["pools"] = [pool for pool in storages.get_storages_info() if pool["status"]]
+        except libvirtError:
+            return None
+        return given
+
     def __str__(self):
         return self.name