fix(quota): a disk whose size is unknown is not counted as 0 GiB

A disk of the user's VMs whose size could not be read (its volume not in a
pool, the pool stopped) counted as 0 GiB, so a disk-limited user's increase
could pass over the limit. A disk no pool holds now has the size libvirt
reads from the file or device (blockInfo, also for a shut-off VM); one whose
size is still unknown makes the usage unverified and the increase is
refused, as when a host cannot be reached. Users without a disk limit are
not affected.

vrtManager/tests/test_read_cache.py

@@ -25,6 +25,9 @@ class FakeDomain:
         self.reads.append(flags)
         return f"<domain flags='{flags}' read='{len(self.reads)}'/>"
 
+    def blockInfo(self, dev):  # a disk no pool holds that libvirt cannot read either
+        raise libvirtError("cannot read")
+
 
 class FakeConn:
     def __init__(self):