KVM: s390x: implement KVM_CAP_SET_GUEST_DEBUG2

Define KVM_GUESTDBG_VALID_MASK and use it to implement this capabiity.
Compile tested only.

Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
Message-Id: <20210401135451.1004564-6-mlevitsk@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>