KVM: s390: Add helper function for setting condition code

Introduced a helper function for setting the CC in the
guest PSW to improve the readability of the code.

Signed-off-by: Thomas Huth <thuth@linux.vnet.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>