arm64/vmm: Handle VM_EXITCODE_SUSPENDED

This is required for bhyve reboot to work. In particular, unless we
suspend vcpu threads here, vm_reinit() will fail with EBUSY.

The implementation is copied from amd64; in the not-too-distant future
the amd64 and arm64 copies of vmm.c and vmm_dev.c will be merged, so
for now it's useful to minimize diffs between amd64 and arm64.

Reviewed by: corvink, andrew
MFC after: 2 weeks
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D44934