vmdk: Widen before shifting 32 bit header field

Coverity spotted this.

The field is 32 bits, but if it's possible to overflow in 32 bit
left shift.

Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>