vm/qemu: fix {{TCP_PORT}} replacement
vm/qemu: fix {{TCP_PORT}} replacement

compiler says:

vm/qemu/qemu.go:501:49: conversion from int to string yields a string
	of one rune, not a string of digits (did you mean fmt.Sprint(x)?)

google/syzkaller,google/syzkaller,google/syzkaller,xairy/syzkaller,xairy/syzkaller,google/syzkaller,xairy/syzkaller,xairy/syzkaller,xairy/syzkaller,xairy/syzkaller,google/syzkaller,google/syzkaller,xairy/syzkaller,google/syzkaller