Add 'info pg' command to monitor

[Don Porter]

Hi all,

I am a CS professor (and, newly, a second-time contributor). I have
been using qemu in my courses for over a decade, especially a course
that asks students to write major pieces of an OS kernel from starter
code.

I have some patches, originally from Austin Clements at MIT, that I
have found useful over the years and that may be useful to others.  It
would also be nice not to have to build a custom qemu each semester.  I
have cleared upstreaming these with Austin, the original author.

This patch set adds an 'info pg' command to the monitor, which prints
a nicer view of the page tables.  A project in my graduate OS course
involves implementing x86 page table support, and my students have
found this helpful for debugging.

Thank you in advance for your time,
Don Porter