[nova] riscv64 emulation support

Hi Nova Team We've had some initial support for enabling RISCV64 support in Nova in a review for sometime which I refreshed this week: https://review.opendev.org/c/openstack/nova/+/889137 Sean suggested that this really needs a blueprint and discussion at the next Nova meeting (which I can't make) so proposing discussion via ML instead: https://blueprints.launchpad.net/nova/+spec/riscv64-emulation-support I'm also going to look at the nova-emulation job to see if that can be extended to cover RISCV64 as well - there are a few nuances with dependent packages (opensbi) in Ubuntu 24.04 but I think we can come up with something workable for testing. Review and comments are appreciated. Thanks James