6.1 Regression: machine pflash parsing

Since the following commit:

commit d8fb7d0969d5c32b3d1b9e20b63ec6c0abe80be4
Author: Paolo Bonzini <pbonzini@redhat.com>
Date:   Thu May 13 11:28:34 2021 -0400

    vl: switch -M parsing to keyval

A VM with the following XML:

  <os>
    <type arch='x86_64' machine='pc'>hvm</type>
    <loader readonly='yes' type='pflash'>/usr/share/edk2/ovmf/OVMF_CODE.fd</loader>
  </os>

And generating the simplified QEMU command line:

qemu-system-x86_64 \
-blockdev '{"driver":"file","filename":"/usr/share/edk2/ovmf/OVMF_CODE.fd","node-name":"pflash0-storage","auto-read-only":true,"discard":"unmap"}' \
-blockdev '{"node-name":"pflash0-format","read-only":true,"driver":"raw","file":"pflash0-storage"}' \
-machine pc,pflash0=pflash0-format

Fails to boot with the error:

qemu-system-x86_64: Parameter 'drive' is missing