qemu: fix formatting of pflash readonly attribute

qemu: fix formatting of pflash readonly attribute

When the <loader> had an explicit readonly='no' attribute we
accidentally still marked the plfash as readonly due to the
bad conversion from virTristateBool to bool. This was missed
because the test cases run with no capabilities set and thus
are validated the -drive approach for pflash configuration,
not the -blockdev approach.

This affected the following config:

  <os>
    <loader readonly='no' type='pflash'>/var/lib/libvirt/qemu/nvram/test-bios.fd</loader>
  </os>

for the sake of completeness, we also add a test XML config
with no readonly attribute at all, to demonstrate that the
default for pflash is intended to be r/w.

Reviewed-by: Ján Tomko <jtomko@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>