raw-posix: Detect CDROM via ioctl on linux

Current CDROM detection is hardcoded based on source file name.
Make this smarter on linux by attempting a CDROM specific ioctl.

This makes '-cdrom /dev/sr0' succeed with no media present.

v2:
    Give ioctl check higher priority than filename check.

v3:
    Actually initialize 'prio' variable.
    Check for ioctl success rather than absence of specific failure.

v4:
    Explicitly mention that change is linux specific.

Signed-off-by: Cole Robinson <crobinso@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
