fix panic (sleeping thread) on vmmctl --destroy --vm=* when bhyve is runnning

gistfile1.diff

Index: sys/amd64/vmm/vmm_dev.c
===================================================================
--- sys/amd64/vmm/vmm_dev.c     (revision 238860)
+++ sys/amd64/vmm/vmm_dev.c     (working copy)
@@ -378,7 +378,9 @@
         * XXX must stop virtual machine instances that may be still
         * running and cleanup their state.
         */
+       mtx_lock(&vmmdev_mtx);
        SLIST_REMOVE(&head, sc, vmmdev_softc, link);
+       mtx_unlock(&vmmdev_mtx);
        destroy_dev(sc->cdev);
        vm_destroy(sc->vm);
        free(sc, M_VMMDEV);
@@ -398,12 +400,11 @@
 
        mtx_lock(&vmmdev_mtx);
        sc = vmmdev_lookup(buf);
+       mtx_unlock(&vmmdev_mtx);
        if (sc == NULL) {
-               mtx_unlock(&vmmdev_mtx);
                return (EINVAL);
        }
        vmmdev_destroy(sc);
-       mtx_unlock(&vmmdev_mtx);
        return (0);
 }
 SYSCTL_PROC(_hw_vmm, OID_AUTO, destroy, CTLTYPE_STRING | CTLFLAG_RW,