xen: move hvm save code under common to x86

xen: move hvm save code under common to x86

The code is only used by x86 at this point. Merge common/hvm/save.c
into x86 hvm/save.c. Move the headers and fix up inclusions. Remove
the now empty common/hvm directory.

Also fix some issues while moving:
1. removing trailing spaces;
2. fix multi-line comment;
3. make "i" in hvm_save unsigned int;
4. add some blank lines to separate sections of code;
5. change bool_t to bool.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>