s390-ccw: parse and set boot menu options

s390-ccw: parse and set boot menu options

Set boot menu options for an s390 guest and store them in
the iplb. These options are set via the QEMU command line
option:

    -boot menu=on|off[,splash-time=X]

or via the libvirt domain xml:

    <os>
      <bootmenu enable='yes|no' timeout='X'/>
    </os>

Where X represents some positive integer representing
milliseconds.

Any value set for loadparm will override all boot menu options.
If loadparm=PROMPT, then the menu will be enabled without a
timeout.

Signed-off-by: Collin L. Walling <walling@linux.vnet.ibm.com>
Reviewed-by: Janosch Frank <frankja@linux.vnet.ibm.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>