xen/blktap: fix various checks

xen/blktap: fix various checks

- array indices got checked after having indexed the array already
- several were off by one
- BLKTAP_IOCTL_FREEINTF should not be used on other than the control
  device (or the logic should be changed to that when thus used only
  the respective device can be freed)
- BLKTAP_IOCTL_MINOR can reasonably also be used on non-control
- devices
  (returning that device's minor and ignoring the passed in argument)

Signed-off-by: Jan Beulich <jbeulich@novell.com>