vmm_instruction_emul.c: fix bhyve build

The diagused macro was used to cure a "set but not used" warning. This
broke the build for bhyve since diagused is only defined in the
kernel. Define __diagused when not building the kernel.

Fixes: 5241577a223d ("vmm: fix set but not used warning")
Reported by: Jenkins

(cherry picked from commit d4e8207317ca1827ba2529203d5cc4e67d836fcb)