This makes "qvm rm" more robust and dangerous.

Previously I use usb bus. In that case, sometimes cloud-init can't find the
datasource at boot time because when / is mounted, usb device may not be
ready. When that happened, cloud-init will fall back to None ds, which make
qvm's metadata fail.

When ide bus is used, cdrom is available much earlier. You can check it using
dmesg.

- enabled ghc -O2, I don't know what's the default.
- upload via i234 is faster than direct upload.

do not cache the response in RAM.

only http and local qcow2 image supported.

cirros is not a good image for this test. I don't have ec2 metadata, cirros
starts too slow. Nonetheless, the local disk logic works.

I don't plan to implement it.
It's there for future option extension for "qvm init".

fetch HTTP image and copy image to VM's disk is working.
clone disk with a new size is not implemented yet.

see src/Domain.hs

Now I can config a domain using do notation.