[2.5a2] juju machine allocation with KVM pod networking and constraints behaves improperly

This bug was seen when bootstrapping a controller with Juju, using MAAS 2.5 alpha 2, with spaces defined. There are three different types of problems that need to be addressed:

(1) Bootstrapping causes "no address family in common with the server" error.

# juju bootstrap --constraints "spaces=intranet,private" maas juju
Creating Juju controller "juju" on maas
Looking for packaged Juju agent version 2.4.2 for amd64
Launching controller instance(s) on maas...
ERROR failed to bootstrap model: cannot start bootstrap instance: unexpected: ServerError: 400 Bad Request ({"network": ["Node has no address family in common with the server"]})

(2) Bootstrapping with only a private network constraint (which would create a machine that would be unable to PXE boot) causes an error. MAAS should automatically include a PXE bootable network in this circumstance.

# juju bootstrap --constraints "spaces=private" maas juju
Creating Juju controller "juju" on maas
Looking for packaged Juju agent version 2.4.2 for amd64
Launching controller instance(s) on maas...
ERROR failed to bootstrap model: cannot start bootstrap instance in availability zone "default": failed to acquire node: No available machine matches constraints: [('agent_name', ['6f6d07d0-4787-49ac-85be-ff5c2f5452e6']), ('interfaces', ['0:space=2']), ('mem', ['3584']), ('zone', ['default'])] (resolved to "interfaces=0:space=2 mem=3584.0 zone=default")

(3) Due to the way Juju uses numeric labels when identifying interface constraints, the interfaces which are created based on a request from Juju will have strange, numeric-only names. MAAS should add "eth" to the beginning of each interface, so that the interface names are unambiguous (they could be confused with interface indexes; plus, numeric-only interface names are just plain strange, unless you're talking about switch ports.)