As part of converting -boot to a property with a QAPI type, define
the struct and use it throughout QEMU to access boot configuration.
machine_boot_parse takes care of doing the QemuOpts->QAPI conversion by
hand, for now.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20220414165300.555321-2-pbonzini@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>