builder: don't use setParamIfDefined on bools

The `setDeviceParamIfDefined' function only works on strings, and checks
that the value is not empty before adding the parameter to the request.

In the builder code, in several places, there are some instances of
booleans being converted to strings using `strconv.FormatBool', and then
calling `setDeviceParamIfDefined' with this value, meaning that the
value always gets set in the request.

This commit removes those instances in favour of a manual boolean check,
which can result in adding the parameter to the request only if it is
set to true.

builder/proxmox/common/step_start_vm.go

@@ -261,7 +261,10 @@ func generateProxmoxNetworkAdapters(nics []NICConfig) proxmox.QemuDevices {
 		setDeviceParamIfDefined(devs[idx], "macaddr", nics[idx].MACAddress)
 		setDeviceParamIfDefined(devs[idx], "bridge", nics[idx].Bridge)
 		setDeviceParamIfDefined(devs[idx], "tag", nics[idx].VLANTag)
-		setDeviceParamIfDefined(devs[idx], "firewall", strconv.FormatBool(nics[idx].Firewall))
+
+		if nics[idx].Firewall {
+			devs[idx]["firewall"] = nics[idx].Firewall
+		}
 
 		if nics[idx].MTU > 0 {
 			devs[idx]["mtu"] = nics[idx].MTU
@@ -281,16 +284,18 @@ func generateProxmoxDisks(disks []diskConfig) proxmox.QemuDevices {
 		setDeviceParamIfDefined(devs[idx], "storage", disks[idx].StoragePool)
 		setDeviceParamIfDefined(devs[idx], "cache", disks[idx].CacheMode)
 		setDeviceParamIfDefined(devs[idx], "format", disks[idx].DiskFormat)
-		if devs[idx]["type"] == "scsi" || devs[idx]["type"] == "virtio" {
-			setDeviceParamIfDefined(devs[idx], "iothread", strconv.FormatBool(disks[idx].IOThread))
+
+		if devs[idx]["type"] == "scsi" || devs[idx]["type"] == "virtio" &&
+			disks[idx].IOThread {
+			devs[idx]["iothread"] = "true"
 		}
-		// Proxmox API expects the value of discard to be either "on" or "ignore"
-		setDeviceParamIfDefined(devs[idx], "discard", func() string {
-			if disks[idx].Discard {
-				return "on"
-			}
-			return "ignore"
-		}())
+
+		if disks[idx].Discard {
+			devs[idx]["discard"] = "on"
+		} else {
+			devs[idx]["discard"] = "ignore"
+		}
+
 		// Add SSD flag only if true
 		if disks[idx].SSD {
 			devs[idx]["ssd"] = "1"