[qemu] Use xenstore to configure ioemu block devices.

[qemu] Use xenstore to configure ioemu block devices.
- read ioemu block device config from xenstore
- don't require the ioemu: prefix on block devices any longer
- allow change of media associated with cdrom drives
- replace cdrom= option by :cdrom suffix on regular block device config:
  'file:/root/mytest.iso,hdc:cdrom,r'
- don't create default cdrom drive anymore - to create default empty
  cdrom drive use: ',hdc:cdrom,r'

Signed-off-by: Christian Limpach <Christian.Limpach@xensource.com>