Add prototypes for build-specific tuning:

• vm_prebuild_setup():

Steps to run prior to creating the file-backed
disk image.

• vm_setup():

Steps to run while the file-backed disk image
is mounted.

• vm_postbuild_setup():

Steps to run after the file-backed disk image
is dismounted.

The intention is to override as necessary via build
configuration files, as needed.

Sponsored by: The FreeBSD Foundation