kvm: ppc: Print meaningful message on KVM_CREATE_VM failure

PowerPC has two KVM types (HV, PR) that translate into three kernel
modules:

kvm.ko - common kvm code
kvm_hv.ko - kvm running with MSR_HV=1 or MSR_HV|PR=0 in a nested guest.
kvm_pr.ko - kvm running in usermode MSR_PR=1.

Since the two KVM types can both be running at the same time, this
creates a situation in which it is possible for one or both of the
modules to fail to initialize, leaving the generic one behind. This
leads QEMU to think it can create a guest, but KVM will fail when
calling the type-specific code:

 ioctl(KVM_CREATE_VM) failed: 22 Invalid argument
 qemu-kvm: failed to initialize KVM: Invalid argument

Ideally this would be solved kernel-side, but it might be a while
until we can get rid of one of the modules. So in the meantime this
patch tries to make this less confusing for the end user by adding a
more elucidative message:

 ioctl(KVM_CREATE_VM) failed: 22 Invalid argument
 PPC KVM module is not loaded. Try 'modprobe kvm_hv'.

[dwg: Fixed error in #elif which failed compile on !ppc hosts]
Signed-off-by: Fabiano Rosas <farosas@linux.ibm.com>
Message-Id: <20210722141340.2367905-1-farosas@linux.ibm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

accel/kvm/kvm-all.c

@@ -2397,6 +2397,12 @@ static int kvm_init(MachineState *ms)
                     "- for kernels supporting the vm.allocate_pgste sysctl, "
                     "whether it is enabled\n");
         }
+#elif defined(TARGET_PPC)
+        if (ret == -EINVAL) {
+            fprintf(stderr,
+                    "PPC KVM module is not loaded. Try modprobe kvm_%s.\n",
+                    (type == 2) ? "pr" : "hv");
+        }
 #endif
         goto err;
     }