s390-ccw.img: Reinitialize guessing on reboot

guessed_disk_nature is a static zero variable. As the QEMU ELF
loader does not zero the BSS section, lets do it explicitely here.

This fixes reboot for some corner cases (like FCP flash
devices with logical_block_size=512, physical_block_size=4096)

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Tested-by: Eugene (jno) Dvurechenski <jno@linux.vnet.ibm.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Message-Id: <1425310029-53396-3-git-send-email-borntraeger@de.ibm.com>