The current userboot interface sets this capability using `vm_setup_freebsd_registers_i386` in lib/libvmmapi/libvmmapi_freebsd.c. This patch adds support for loaders to explicitly query and/or set this capability without having to go through the `exec` callback, but set up registers themselves. To simplify setting up a default state, add a callback to wrap `vcpu_reset`.
This change is backwards-compatible as all new callbacks are added to the end of the callback structure.