pc-bios/s390-ccw: fix non-sequential boot entries (enum)

pc-bios/s390-ccw: fix non-sequential boot entries (enum)

zIPL boot menu entries can be non-sequential. Let's account
for this issue for the s390 enumerated boot menu. Since we
can no longer print a range of available entries to the
user, we have to present a list of each available entry.

An example of this menu:

  s390-ccw Enumerated Boot Menu.

   [0] default

   [1]
   [2]
   [7]
   [8]
   [9]
  [11]
  [12]

  Please choose:

Signed-off-by: Collin Walling <walling@linux.ibm.com>
Reported-by: Vasily Gorbik <gor@linux.ibm.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Janosch Frank <frankja@linux.ibm.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>