osbuild: introduce bootloader struct into qemu assembler

osbuild takes a "bootloader" object as an option to the qemu assembler:
3f14ace5c1/assemblers/org.osbuild.qemu (L43)
we don't use it because for x86_64 with enabled legacy support it
defaults to the right value:
3f14ace5c1/assemblers/org.osbuild.qemu (L482)
but in order to gain support for ppc64le we need to introduce this.
Example usage can be found in samples directory:
3f14ace5c1/samples/f30-ppc64le.json (L819)

This change itself does not alter osbuild-composer output.

internal/osbuild/qemu_assembler.go

@@ -9,6 +9,7 @@ import "github.com/google/uuid"
 // containing the indicated partitions. Finally, the image is converted into
 // the target format and stored with the given filename.
 type QEMUAssemblerOptions struct {
+	Bootloader *QEMUBootloader `json:"bootloader,omitempty"`
 	Format     string          `json:"format"`
 	Filename   string          `json:"filename"`
 	Size       uint64          `json:"size"`
@@ -33,6 +34,11 @@ type QEMUFilesystem struct {
 	Mountpoint string `json:"mountpoint"`
 }
 
+type QEMUBootloader struct {
+	Type     string `json:"type"`
+	Platform string `json:"platform"`
+}
+
 func (QEMUAssemblerOptions) isAssemblerOptions() {}
 
 // NewQEMUAssembler creates a new QEMU Assembler object.