s390x/kvm: Report warnings with warn_report(), not error_printf()

kvm_s390_mem_op() can fail in two ways: when !cap_mem_op, it returns
-ENOSYS, and when kvm_vcpu_ioctl() fails, it returns -errno set by
ioctl().  Its caller s390_cpu_virt_mem_rw() recovers from both
failures.

kvm_s390_mem_op() prints "KVM_S390_MEM_OP failed" with error_printf()
in the latter failure mode.  Since this is obviously a warning, use
warn_report().

Perhaps the reporting should be left to the caller.  It could warn on
failure other than -ENOSYS.

Cc: Thomas Huth <thuth@redhat.com>
Cc: qemu-s390x@nongnu.org
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Message-Id: <20190417190641.26814-9-armbru@redhat.com>