raw-posix: Detect legacy floppy via ioctl on linux

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

v2:
    Give ioctl check higher priority than filename check
    s/IDE/legacy/

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>
