virhostcpu: Fix build with clang and newest kernel headers

The most recent environment e.g. present in our Fedora Rawhide builds
fail to build the tree with clang with the following error:

../src/util/virhostcpu.c:1291:25: error: field 'header' with variable sized type 'struct kvm_msrs' not at the end of a struct or class is a GNU extension [-Werror,-Wgnu-variable-sized-type-not-at-end]
        struct kvm_msrs header;
                        ^

The problem seems to be that clang doesn't like the new way the
'entries' field in struct kvm_msrs is declared.

To work around the issue we can simply allocate the variable dynamically
and use the 'entries' member as it was intended to to access the
members.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Jiri Denemark <jdenemar@redhat.com>

src/util/virhostcpu.c

@@ -1287,25 +1287,22 @@ virHostCPUGetMSRFromKVM(unsigned long index,
                         uint64_t *result)
 {
     VIR_AUTOCLOSE fd = -1;
-    struct {
+    g_autofree struct kvm_msrs *msr = g_malloc0(sizeof(struct kvm_msrs) +
-        struct kvm_msrs header;
+                                                sizeof(struct kvm_msr_entry));
-        struct kvm_msr_entry entry;
+    msr->nmsrs = 1;
-    } msr = {
+    msr->entries[0].index = index;
-        .header = { .nmsrs = 1 },
-        .entry = { .index = index },
-    };
 
     if ((fd = open(KVM_DEVICE, O_RDONLY)) < 0) {
         virReportSystemError(errno, _("Unable to open %s"), KVM_DEVICE);
         return -1;
     }
 
-    if (ioctl(fd, KVM_GET_MSRS, &msr) < 0) {
+    if (ioctl(fd, KVM_GET_MSRS, msr) < 0) {
         VIR_DEBUG("Cannot get MSR 0x%lx from KVM", index);
         return 1;
     }
 
-    *result = msr.entry.data;
+    *result = msr->entries[0].data;
     return 0;
 }