The create form and the API took 0 or a negative number for vCPUs,
memory and a new disk's size: the API queued the job (202) and it
failed later in libvirt; the page did the same, its formnovalidate
skipping the browser's min. The create model's fields now refuse
values below 1, so the form shows the error beside its field and the
API answers 400 naming it.
The form also checks the MAC addresses as the API and the job do (one
per NIC at most, each a MAC address), so a bad one is an error on the
form instead of a job that fails.
Closes#175
