builder/qemu: add use_default_display option

If set, won't set `-display dsl` as a default argument. Useful for osx
compatibility.

builder/qemu/builder.go

@@ -104,6 +104,7 @@ type Config struct {
 	ShutdownCommand   string     `mapstructure:"shutdown_command"`
 	SSHHostPortMin    uint       `mapstructure:"ssh_host_port_min"`
 	SSHHostPortMax    uint       `mapstructure:"ssh_host_port_max"`
+	UseDefaultDisplay bool       `mapstructure:"use_default_display"`
 	VNCBindAddress    string     `mapstructure:"vnc_bind_address"`
 	VNCPortMin        uint       `mapstructure:"vnc_port_min"`
 	VNCPortMax        uint       `mapstructure:"vnc_port_max"`

builder/qemu/step_run.go

@@ -124,7 +124,9 @@ func getCommandArgs(bootDrive string, state multistep.StateBag) ([]string, error
 		}
 	} else {
 		if qemuMajor >= 2 {
+			if !config.UseDefaultDisplay {
 				defaultArgs["-display"] = "sdl"
+			}
 		} else {
 			ui.Message("WARNING: The version of qemu  on your host doesn't support display mode.\n" +
 				"The display parameter will be ignored.")

website/source/docs/builders/qemu.html.md

@@ -236,6 +236,10 @@ Linux server and have not enabled X11 forwarding (`ssh -X`).
     switch/value pairs. Any value specified as an empty string is ignored. All
     values after the switch are concatenated with no separator.
 
+-   `use_default_display` (boolean) - If true, do not pass a `-display` option
+    to qemu, allowing it to choose the default. This may be needed when running
+    under OS X.
+
 \~> **Warning:** The qemu command line allows extreme flexibility, so beware
 of conflicting arguments causing failures of your run. For instance, using
 --no-acpi could break the ability to send power signal type commands (e.g.,