DVD emulation: make seek timing more accurate.

Based on real measurements of a Gamecube disk on a Wii.  (If anyone else
is interested in running more tests, I can upload my code; it's a mess at
the moment.)

This isn't a complete model, but it's a step in the right direction:
in Metroid Prime, the loading times are sane, and I think I managed to do it
without breaking anything.
DVDInterface: tweak buffered reads to be a bit faster.

A read finishes when the result is complete in the drive's buffer, not
main memory.
DVDInterface: code cleanup.
Merge pull request #327 from magumagu/dvd-seek-time

DVD emulation: make seek timing more accurate.