Use virFileIsCDROM to detect whether a block device is a cdrom drive and
store it in virStorageSource. This will be necessary to correctly create
the 'host_cdrom' backend in qemu when using -blockdev.

We assume that host_cdrom makes only sense when used directly as a raw
image, but if a backing chain would be put in front of it, libvirt will
use 'host_device' in that case.
Signed-off-by: NPeter Krempa <pkrempa@redhat.com>
Reviewed-by: NJohn Ferlan <jferlan@redhat.com>