kernel/vtnet: Replace a kmalloc by contigmalloc.

* To get physically contiguous memory, we have to use contigmalloc instead
  of kmalloc.

* Use a 2 byte alignment to make sure that a 2 byte local variable is
  stored in physically contiguous memory.

Dragonfly-bug: <http://bugs.dragonflybsd.org/issues/2666>
kernel/acpi: Improve a message a bit.