block: allow creation of fixed vhdx images

When trying to create a fixed vhd image qemu-img will return the
following error:

 qemu-img: test.vhdx: Could not create image: Cannot allocate memory

This happens because of a incorrect check in vhdx.c. Specifficaly,
in vhdx_create_bat(), after allocating memory for the BAT entry,
there is a check to determine if the allocation was unsuccsessful.
The error comes from the fact that it checks if s->bat isn't NULL,
which is true in case of succsessful allocation,  and exits with
error ENOMEM.
Signed-off-by: NAdelina Tuvenie <atuvenie@cloudbasesolutions.com>
Acked-by: NKevin Wolf <kwolf@redhat.com>
Signed-off-by: NMichael Tokarev <mjt@tls.msk.ru>

block/vhdx.c

@@ -1593,7 +1593,7 @@ static int vhdx_create_bat(BlockDriverState *bs, BDRVVHDXState *s,
                 bdrv_has_zero_init(bs) == 0) {
         /* for a fixed file, the default BAT entry is not zero */
         s->bat = g_try_malloc0(length);
-        if (length && s->bat != NULL) {
+        if (length && s->bat == NULL) {
             ret = -ENOMEM;
             goto exit;
         }