fix(instances): a resize keeps a VM's maximum vCPUs above the host's CPUs

The Resize tab offered vCPU counts only up to the host's logical CPUs.
A VM whose maximum was higher (4 on a 2-CPU host, say) found no option
for it, so the Maximum list showed 1, and a resize of the shut-off VM
lowered its maximum to that without a word.

get_max_cpus() now takes the VM's own maximum as a floor, and the
Instance's vcpu_range passes it. The host's CPU count shown on the page
and used by the clone form is read from the host alone, not from that
range.

instances/models.py

@@ -117,7 +117,8 @@ class Instance(models.Model):
 
     @cached_property
     def vcpu_range(self):
-        return self.proxy.get_max_cpus()
+        """The vCPU counts the Resize tab offers, the VM's own maximum among them."""
+        return self.proxy.get_max_cpus(at_least=self.config_vcpu)
 
     @cached_property
     def cur_vcpu(self):