fix(instances): the Resize tab changes only what was asked

Disk sizes were pre-filled rounded to whole GiB, so a 3.5 GiB disk showed
4 and was grown, for good, whenever another disk was resized; an empty
field or a disk outside a pool was a 500. A disk's field is now empty with
its size as the placeholder: an empty field leaves the disk alone, a
larger whole number grows it, anything else is refused with a message.

vCPUs change only on a shut-off VM, also on a direct request: a running
VM got them at its next start, past the quota counted against its live
count. Current vCPUs or memory above the maximum are refused instead of
failing (and dropping vCPU hot-plug) or being capped by libvirt. A running
VM's maximum memory select is disabled, since its maximum is kept, and a
custom memory value is sent only while its box is ticked.

instances/templates/instance.html

@@ -320,6 +320,8 @@
             var container  = $(this).closest('.js-custom__container');
             var toggles    = container.find('.js-custom__toggle');
             toggles.toggle();
+            // a custom value is sent only while its box is ticked
+            container.find('input.js-custom__toggle').prop('disabled', !this.checked);
         });
     });
 </script>