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: Adelina Tuvenie <atuvenie@cloudbasesolutions.com>
Acked-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
