bhyve: Handle exits due to membar accesses

[kib]

Summary

PR: 250802

[grehan]

This is incorrect: there should never be an exit for a membar access by a guest, hence the assert.

The membar is directly mapped into guest address space - see vmm_mem.c:vmm_mmio_alloc()

[kib]

In D27092#604735, @grehan wrote:

This is incorrect: there should never be an exit for a membar access by a guest, hence the assert.

The membar is directly mapped into guest address space - see vmm_mem.c:vmm_mmio_alloc()

May be, I do not know yet. Please see the referenced PR https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=250802, I put this patch out for the user to try. Phab automatically assigned reviewers.