make bhyve device headers independent
The headers required for interacting directly with the bhyve kernel vmm devices (
/dev/vmm/<instance>) could depend less on other system headers, making it easier to consume their contents without dragging in a bunch of other includes, either from the kernel or the bhyve compat/contrib areas.
This applies to both
vmm_dev.h, both of which are required for bhyve device ioctls.