vhdx: Check for 4 GB maximum log size on creation

It's unclear what the real maximum is, but we use an uint32_t to store
the log size in vhdx_co_create(), so we should check that the given
value fits in 32 bits.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>