fix(instances): VM list columns, hosts for viewers, flavor units

- On a narrow screen the grouped list hid the User, VCPU and Memory
  headings but not the VMs' cells, so values sat under the wrong headings;
  the cells now hide with their headings. Memory says MiB.
- A user with the view_instances permission sees each VM's host in the
  list, as the host alerts already name them; the host page link stays a
  superuser's.
- A flavor's memory and disk are labelled MiB and GiB, and each value is at
  least 1, so "4" is no longer saved as a 4 MiB preset by mistake.

instances/forms.py

@@ -10,6 +10,11 @@ from .utils import nic_macs
 
 
 class FlavorForm(forms.ModelForm):
+    # the units the create form fills in from a flavor
+    memory = forms.IntegerField(label=_("Memory (MiB)"), min_value=1)
+    vcpu = forms.IntegerField(label=_("vCPUs"), min_value=1)
+    disk = forms.IntegerField(label=_("Disk (GiB)"), min_value=1)
+
     class Meta:
         model = Flavor
         fields = "__all__"