libkvm: support access to vmm guest memory, allow writes to fwmem and vmm

[avg]

Summary

This change consists of two parts:

• allow libkvm to recognize /dev/vmm/* character devices as devices that provide access to the physical memory of a system (similarly to /dev/fwmem*)
• allow libkvm to recognize that /dev/vmm/* and /dev/fwmem* devices provide access to the physical memory of live remote systems and, thus, the memory is writable

As a result, it should be possible to run commands like
$ kgdb -w /path/to/kernel /dev/fwmem0.0
$ kgdb /path/to/kernel /dev/vmm/guest

And other commands that employ libkvm to access system memory images.

[kib]

lib/libkvm/kvm.c

185 ↗

(On Diff #22618)

Shouldn't this depend on flags ? If O_RDONLY is used, writeable seems to be not true.

[avg]

lib/libkvm/kvm.c

185 ↗

(On Diff #22618)

So, 'writable' should be read as 'potentially writable'. If a device is opened read-only, then writing to it will fail anyway and an appropriate error would be reported.

[jhb]

The writable name is a bit confusing as Kostantin noted, but I don't have a better suggestion.