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.

vrtManager/storage.py

@@ -14,7 +14,7 @@ class wvmStorages(wvmConnect):
             stg_status = stg.isActive()
             stg_type = util.get_xml_path(stg.XMLDesc(0), "/pool/@type")
             stg_vol = len(stg.listVolumes()) if stg_status else None
-            stg_size = stg.info()[1]
+            _state, stg_size, _allocation, stg_free = stg.info()
             storages.append(
                 {
                     "name": pool,
@@ -22,6 +22,7 @@ class wvmStorages(wvmConnect):
                     "type": stg_type,
                     "volumes": stg_vol,
                     "size": stg_size,
+                    "free": stg_free,
                 }
             )
         return storages