libxc: reorganize domain builder guest memory allocator

libxc: reorganize domain builder guest memory allocator

Guest memory allocation in the domain builder of libxc is done via
virtual addresses only. In order to be able to support preallocated
areas not virtually mapped reorganize the memory allocator to keep
track of allocated pages globally and in allocated segments.

This requires an interface change of the allocate callback of the
domain builder which currently is using the last mapped virtual
address as a parameter. This is no problem as the only user of this
callback is stubdom/grub/kexec.c using this virtual address to
calculate the last used pfn.

Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>