What constitutes a house for villagers?

Solution 1:

You are using version 1.1, and new villager behaviour, including populating player-built houses, is not implemented until the 12w07a snapshot. This will be included in the 1.2 release.

If you are running a new enough version, a house is a structure:

• made of planks, with walls, floor and ceiling.
• with a door.

Solution 2:

i've done it! it's a little involved, and might not be exactly what you wanted, but here it goes.
according to the wiki, a Villager's Brain NBT tag seems to do the trick. here's the syntax i used that seems to work after 10 tests;

/summon minecraft:villager ~ ~ ~ {Brain:[{home:[{posX:*,posY:*,posZ:*}],dim:*}]}

obviously y'all know what the summon, villager etc. does. lemme explain the NBT;

Brain being the parent is a little self explanatory- it's everything the Villager needs to know it should do.
home is the tag that says the location of the bed. i used the bottom portion of the beds, and it seemed to work each time, so safe to assume that's the master block. you'll need specific coordinates in posX, posY, and posZ instead of "", since i only used "" as a wildcard for the explanation.
dim is the dimension ID that the bed is located in-- 0 for the Overworld.

this is up-to-date for 1.13, and i have not tried this in any snapshots. if anyone wants to test this further in case my results were freak coincidences, or whatever, please feel free to do so.