Solve Race Condition in ATA

[dthain]

When issuing an ata/atapi read/write, the ata driver should generally call process_wait() to wait for an interrupt to arrive. However, on "fast" virtual hardware, the interrupt is often delivered even before process_wait() has been called. Hence, the current code has these waits commented out, and just does a busy-wait on drive status.

Fix the race condition by blocking interrupts in the critical section, so that we can properly handle "slow" real hardware without busy waiting.

See PR #72 for an instance of the problem and PR #82 for the temporary workaround.