target/s390x/kvm: Refactor AP functionalities

kvm_s390_set_attr() is a misleading name as it only sets attributes for
the KVM_S390_VM_CRYPTO group. Therefore, rename it to
kvm_s390_set_crypto_attr().

Add new functions ap_available() and ap_enabled() to avoid code
duplication later.

Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Michael Mueller <mimu@linux.ibm.com>
Signed-off-by: Steffen Eiden <seiden@linux.ibm.com>
Message-ID: <20230823142219.1046522-5-seiden@linux.ibm.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>