QEMU is not correctly detecting host CDs

QEMU's block layer contains code for detecting and using ioctls when real CD-ROM host devices are attached.

This detection is not working in some host OSes while bad implemented on anothers.

E.g., in Linux host qemu -cdrom /dev/sr0 is not detecting it as a CD-ROM
E.g., in Mac OS X host qemu asks the kernel to enumerate optical devices and the compares it to the constant string "/dev/cdrom". This is useless, that enumeration is just enough, and "/dev/cdrom" will NEVER exist in Mac OS X unless manually created by the user.