iotests: Make 150 use qemu-img map instead of du

The actual on-disk size of a file does not only depend on factors qemu
can control. Thus, we should not depend on this to determine whether a
file has indeed been fully allocated. Instead, use qemu-img map and hope
that if an area is referenced, it is indeed allocated, too.

Also, limit the supported image formats to raw and qcow2 because the
actual qemu-img map output may depend on the image format.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Tested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>