bhyve: fix resume for vms with guest_ncpus > 1

This error occurs because vm->vcpu[1] has not been allocated yet when
vm_snapshot_vm() is called.

To fix this, move spinup_vcpu() before restore code.

Reviewed by: corvink, markj
MFC after: 2 weeks
Sponsored by: vStack
Differential Revision: https://reviews.freebsd.org/D38477