bhyve: Use the proper type for string literals

No functional change intended.

MFC after:	1 week

usr.sbin/bhyve/ipc.h

@@ -34,7 +34,7 @@
 #include <sys/nv.h>
 
 struct ipc_command {
-	char *name;
+	const char *name;
 	int (*handler)(struct vmctx *ctx, const nvlist_t *nvl);
 };
 

usr.sbin/bhyve/usb_emul.h

@@ -49,7 +49,7 @@ struct vm_snapshot_meta;
 
 /* Device emulation handlers */
 struct usb_devemu {
-	char	*ue_emu;	/* name of device emulation */
+	const char *ue_emu;	/* name of device emulation */
 	int	ue_usbver;	/* usb version: 2 or 3 */
 	int	ue_usbspeed;	/* usb device speed */