tools/libxencall: enforce proper alignment of hypercall buffers
xencall_alloc_buffer() is used throughout Xen tools for allocating
hypercall buffers. Allocation is done at page granularity. For simple
administration each allocated set of pages contains a small header
holding the number of pages of that set. The hypercall buffer is
located directly after the 4 byte sized header, leading to a wrong
alignment for e.g. pointers.
Repair that by using a 16 byte sized header enforcing the same
alignment as malloc().