kvm: Print MSR information if KVM_{GET,SET}_MSRS failed

When a KVM_{GET,SET}_MSRS ioctl() fails, it is difficult to find
out which MSR caused the problem. Print an error message for
debugging, before we trigger the (ret == cpu->kvm_msr_buf->nmsrs)
assert.
Suggested-by: NDr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: NEduardo Habkost <ehabkost@redhat.com>
Message-Id: <20170309194634.28457-1-ehabkost@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

target/i386/kvm.c

@@ -1824,6 +1824,12 @@ static int kvm_put_msrs(X86CPU *cpu, int level)
         return ret;
     }
 
+    if (ret < cpu->kvm_msr_buf->nmsrs) {
+        struct kvm_msr_entry *e = &cpu->kvm_msr_buf->entries[ret];
+        error_report("error: failed to set MSR 0x%" PRIx32 " to 0x%" PRIx64,
+                     (uint32_t)e->index, (uint64_t)e->data);
+    }
+
     assert(ret == cpu->kvm_msr_buf->nmsrs);
     return 0;
 }
@@ -2189,6 +2195,12 @@ static int kvm_get_msrs(X86CPU *cpu)
         return ret;
     }
 
+    if (ret < cpu->kvm_msr_buf->nmsrs) {
+        struct kvm_msr_entry *e = &cpu->kvm_msr_buf->entries[ret];
+        error_report("error: failed to get MSR 0x%" PRIx32,
+                     (uint32_t)e->index);
+    }
+
     assert(ret == cpu->kvm_msr_buf->nmsrs);
     /*
      * MTRR masks: Each mask consists of 5 parts