vpc: fix return value check for blk_pwrite

bdrv_pwrite_sync used to return zero or negative error, while blk_pwrite returns
the number of written bytes when successful.  This caused VPC image creation
to fail spectacularly: it wrote the first 512 bytes, and then exited immediately
because of the non-zero answer from blk_pwrite.  But the truly spectacular part
is that it returns a positive value (the 512 that blk_pwrite returned) causing
everyone to believe that it succeeded.

This fixes qemu-iotests with vpc format.

Fixes: b8f45cdf
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>