qemu: command: Use 'qemuBuildChrChardevCommand' to build TPM backend

qemu: command: Use 'qemuBuildChrChardevCommand' to build TPM backend

Since the backend of the TPM is a chardev we can use the common helper
to instantiate it.

This commit also ensures proper ordering so that the backend chardev is
formatted before it's being referenced.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>