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

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

Reviewed by: kib, jhb
MFC after: 2 weeks
Relnotes: yes
Sponsored by: Panzura
Differential Revision: https://reviews.freebsd.org/D8679