bhyve: initialize register value

In case of an error in a code pattern like

```
uint64_t val;
error = memread(vcpu, gpa, &val, 1, arg);
error = vie_update_register(vcpu, reg, val, size);
```

uninitialized stack data would be used.

Reported by:    Synacktiv
Reviewed by:	markj
Security:       HYP-21
Sponsored by:   The Alpha-Omega Project
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D46107

sys/amd64/vmm/vmm_ioport.c

@@ -101,7 +101,7 @@ static int
 emulate_inout_port(struct vcpu *vcpu, struct vm_exit *vmexit, bool *retu)
 {
 	ioport_handler_func_t handler;
-	uint32_t mask, val;
+	uint32_t mask, val = 0;
 	int error;
 
 	/*