Plugin Address Translations Inconsistent/Incorrect?

[Aaron Lindsay <aaron@os.amperecomputing.com>]

Hello,

I've been doing some more work with plugins and found something I didn't
expect with regards to address translation.

If I call (inside a memory callback):

`uint64_t pa = qemu_plugin_hwaddr_device_offset(hwaddr);`

I see that `pa` takes the value 0xe0e58760. If, however, I plumb
`cpu_get_phys_page_debug` through to the plugin interface and call it
like:

`pa = cpu_get_phys_page_debug(current_cpu, va);`

I see it takes the value 0x120e58760.

I notice that 0x120e58760-0xe0e58760 is exactly one gigabyte, which is
also the offset of the beginning of RAM for the 'virt' AArch64 machine
I'm using. Furthermore, I see the name of the plugin function includes
"device_offset", so perhaps this discrepancy is by design. However, it
seems awkward to not be able to get a true physical address.

I've done some digging and found that inside `qemu_ram_addr_from_host`
(called by `qemu_plugin_hwaddr_device_offset`), `block->mr->addr`
appears to hold the offset of the beginning of RAM. 

Do you think it would be reasonable to modify
`qemu_plugin_hwaddr_device_offset` to add the beginning of the RAM block
or otherwise return the true physical address (or at least expose a way
to find the beginning of it through the plugin interface)?

Thanks!

-Aaron