tools/libxl: Correctly align the ACPI tables

tools/libxl: Correctly align the ACPI tables

The memory allocator currently calculates alignment in libxl's virtual
address space, rather than guest physical address space. This results
in the FACS being commonly misaligned.

Furthermore, the allocator has several other bugs.

The opencoded align-up calculation is currently susceptible to a bug
that occurs in the corner case that the buffer is already aligned to
begin with. In that case, an align-sized memory hole is introduced.

The while loop is dead logic because its effects are entirely and
unconditionally overwritten immediately after it.

Rework the memory allocator to align in guest physical address space
instead of libxl's virtual memory and improve the calculation, drop
errant extra page in allocated buffer for ACPI tables, and give some
of the variables better names/types.

Fixes: 14c0d328da2b ("libxl/acpi: Build ACPI tables for HVMlite guests")
Signed-off-by: Kevin Stefanov <kevin.stefanov@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Ian Jackson <iwj@xenproject.org>