bhyve: Allocate struct vm_exit on the stack in vm_loop.

The global vmexit[] array is no longer needed to smuggle the rip
value from fbsdrun_addcpu() to vm_loop().

Reviewed by: corvink, markj
Differential Revision: https://reviews.freebsd.org/D37644

(cherry picked from commit a20c00c60e1c022f77a7bc638f29d3d6d9456953)