Add an option flag to allow using a pflash for firmware

Since the merge of hashicorp#43 any script with a firmware setting, will
use an `-drive if=pflash` option instead of `-bios` when calling
QEMU, which has been shown to be problematic.

Implement a new option `pflash` instead, that could be set to
true for when that would be preferred (ex: when using a custom
firmware with variables), but use the old interface by default.

Fixes: hashicorp#66

builder/qemu/config.go

@@ -109,12 +109,18 @@ type Config struct {
 	// The number of cpus to use when building the VM.
 	//  The default is `1` CPU.
 	CpuCount int `mapstructure:"cpus" required:"false"`
-	// The firmware file to be used by QEMU, which is to be set by the -bios
-	// option of QEMU. Particularly, this option can be set to use EFI instead
-	// of BIOS, by using "OVMF.fd" from OpenFirmware.
-	// If unset, no -bios option is passed to QEMU, using the default of QEMU.
+	// The firmware file to be used by QEMU
+	// this option could be set to use EFI instead of BIOS,
+	// by using "OVMF.fd" from OpenFirmware, for example.
+	// If unset, QEMU will load its default firmware.
 	// Also see the QEMU documentation.
 	Firmware string `mapstructure:"firmware" required:"false"`
+	// If a firmware file option was provided, this option can be
+	// used to change how qemu will get it.
+	// If false (the default), then the firmware is provided through
+	// the -bios option, but if true, a pflash drive will be used
+	// instead.
+	PFlash bool `mapstructure:"use_pflash" required:"false"`
 	// The interface to use for the disk. Allowed values include any of `ide`,
 	// `sata`, `scsi`, `virtio` or `virtio-scsi`^\*. Note also that any boot
 	// commands or kickstart type scripts must have proper adjustments for

builder/qemu/step_run.go

@@ -104,6 +104,11 @@ func (s *stepRun) getDefaultArgs(config *Config, state multistep.StateBag) map[s
 			config.MachineType, config.Accelerator)
 	}
 
+	// Firmware
+	if config.Firmware != "" && !config.PFlash {
+		defaultArgs["-bios"] = config.Firmware
+	}
+
 	// Configure "-netdev" arguments
 	defaultArgs["-netdev"] = fmt.Sprintf("bridge,id=user.0,br=%s", config.NetBridge)
 	if config.NetBridge == "" {
@@ -275,7 +280,7 @@ func (s *stepRun) getDeviceAndDriveArgs(config *Config, state multistep.StateBag
 		}
 	}
 	// Firmware
-	if config.Firmware != "" {
+	if config.Firmware != "" && config.PFlash {
 		driveArgs = append(driveArgs, fmt.Sprintf("file=%s,if=pflash,format=raw,readonly=on", config.Firmware))
 	}
 

docs-partials/builder/qemu/Config-not-required.mdx

@@ -35,12 +35,18 @@
 - `cpus` (int) - The number of cpus to use when building the VM.
    The default is `1` CPU.
 
-- `firmware` (string) - The firmware file to be used by QEMU, which is to be set by the -bios
-  option of QEMU. Particularly, this option can be set to use EFI instead
-  of BIOS, by using "OVMF.fd" from OpenFirmware.
-  If unset, no -bios option is passed to QEMU, using the default of QEMU.
+- `firmware` (string) - The firmware file to be used by QEMU
+  this option could be set to use EFI instead of BIOS,
+  by using "OVMF.fd" from OpenFirmware, for example.
+  If unset, QEMU will load its default firmware.
   Also see the QEMU documentation.
 
+- `use_pflash` (bool) - If a firmware file option was provided, this option can be
+  used to change how qemu will get it.
+  If false (the default), then the firmware is provided through
+  the -bios option, but if true, a pflash drive will be used
+  instead.
+
 - `disk_interface` (string) - The interface to use for the disk. Allowed values include any of `ide`,
   `sata`, `scsi`, `virtio` or `virtio-scsi`^\*. Note also that any boot
   commands or kickstart type scripts must have proper adjustments for