Add proper QEMU Machine Protocol commands for reading/writing memory and getting vmcoreinfo

For reading memory, our QMP support uses `xp` via the `human-monitor-command` QMP command, which the QEMU documentation [strongly discourages](https://www.qemu.org/docs/master/interop/qemu-qmp-ref.html#command-QMP-misc.human-monitor-command). On top of that, it requires us to parse a hexdump that is intended for human consumption. For getting vmcoreinfo, we're using `dump-guest-memory`, which only works locally and also requires us to use `info mtree -f` via `human-monitor-command` to find a valid RAM physical address. It'd be really nice to have proper QMP commands for: - [ ] Reading memory (probably still in hex since the protocol is JSON, but at least as a simple hex string instead of all of the extra labels, spaces, etc.). - [ ] Writing memory. - [ ] Dumping vmcoreinfo.