exec: check kvm mmu notifiers earlier

Move kvm mmu notifiers check before calling file_ram_alloc(), with the
other xen precondition. (file_ram_alloc() will be reused in other cases
than -mem-path).
Signed-off-by: NMarc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20170602141229.15326-2-marcandre.lureau@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

exec.c

@@ -1495,12 +1495,6 @@ static void *file_ram_alloc(RAMBlock *block,
     int fd = -1;
     int64_t file_size;
 
-    if (kvm_enabled() && !kvm_has_sync_mmu()) {
-        error_setg(errp,
-                   "host lacks kvm mmu notifiers, -mem-path unsupported");
-        return NULL;
-    }
-
     for (;;) {
         fd = open(path, O_RDWR);
         if (fd >= 0) {
@@ -1943,6 +1937,12 @@ RAMBlock *qemu_ram_alloc_from_file(ram_addr_t size, MemoryRegion *mr,
         return NULL;
     }
 
+    if (kvm_enabled() && !kvm_has_sync_mmu()) {
+        error_setg(errp,
+                   "host lacks kvm mmu notifiers, -mem-path unsupported");
+        return NULL;
+    }
+
     if (phys_mem_alloc != qemu_anon_ram_alloc) {
         /*
          * file_ram_alloc() needs to allocate just like