add: original codes of Definitive Guide to the Xen Hypervisor
ch2: fix Makefile and add a 'run' rule.

We need to explicitly specify the x86-32 flag to compiler because
the assembly code is 32-bit and the compiler might be 64-bit.

'make run' is very convenient for testing.
ch2: fix domain_config, using full path for kernel

Not sure why it requires full path.
ch2: fix guest type to support PAE.

Xen 4 does not support xen-3.0-x86_32 guest any more, and it requires the
guest supports PAE if the guest is 32-bit.
.gitignore: add more rules into .gitignore
ch2: prepare for x64 arch

Separate arch-dep files, flags.
ch2: add x86_64 support

Files are modified from Mini-OS.
ch6: workable in x86_32

Basically, the main modifications follow the changes in the
chapter2. In addition, I added an additional CFLAG
"-fno-stack-protector" to avoid error from "__stack_chk_fail
undefined reference."
ch6: import x64 files and corresponding modification of Makefile

hypercall-x86_64.h : xen-4.2.1/extras/mini-os/include/x86/x86_64
bootstrap.x86_64.S : xen-4.2.1/extras/mini-os/arch/x86/x86_64.S
loader-x86_64.lds  : ../chapter2/x86_64.lds
NOTE: not yet compiled.
ch6: working in x86_64

Include correct header file. API of HYPERVISOR_set_callbacks is changed.
Implementation of first_bit() and xchg require changing.
Added difinition of pte_t and __pte(x), maybe these 2 macro should be
defined in x86_mm.h.
ch7: workable in x86_32

Same modification as in ch6.
ch7: copy x64 files from chapter6
ch7: fix: console input does not work in x86_64

A stupid typo (x96_64) make the first_bit work incorrectly.
Also add #else macro to prevent such kind of error.
ch6: fix the same typo as last commit
ch8: compiled in x86_32 but not yet worked

The guest crashed without any messages on console. However, xm dmesg
showed some segmentation faults from guest domain.
ch8: fix VM terminated immediately

We should not directly return from start_kernel(). However,
using HYPERVISOR_sched_op with SCHEDOP_block or SCHEDOP_yield
does not work, either..