Only leave the bootprio code in setup_drive_hd, like in setup_drive_cdrom.
This is a preparatory step; later, the SCSI code in usb_msc_init will
become entirely generic.
Also, the returned number of sectors is off by one. This will become
more important when CHS translation is added later.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>