Implement cdrom disk emulation at high-level instead of in low-level ATA.

Implement cdrom disk emulation at high-level instead of in low-level ATA.

Add a 2K buffer to the ebda to allow for cdrom 512 byte vs 2048 byte
    sector emulation.
For unaliagned cdemu reads, just make multiple cdrom reads instead of
    using ata specific code for short reads.
Also, define cdemu virtual sectors using struct chs_s, and update
    legacy_lba() func to take pointer to a chs_s struct.