libxl: fix a variable underflow in libxl_wait_for_free_memory

libxl: fix a variable underflow in libxl_wait_for_free_memory

When xl is called to create a domU and there is not enough memory available,
then the autoballooning is called to extract memory from dom0. During the
ballooning a loop in libxl_wait_for_free_memory() waits unless enough memory
is available to create the domU.

But because of a variable-underflow the loop can finish too soon and xl
finally aborts with the message:

xc: error: panic: xc_dom_boot.c:161: xc_dom_boot_mem_init: can't allocate low memory for domain: Out of memory
libxl: error: libxl_dom.c:430:libxl__build_pv: xc_dom_boot_mem_init failed: Device or resource busy
libxl: error: libxl_create.c:901:domcreate_rebuild_done: cannot (re-)build domain: -3

The variable-underflow happens when freemem_slack is larger then
info.free_pages*4, because the solution of this operation is converted
implicit to a unsigned int to match the type of memory_kb.

Add a extra check for this condition to solve the problem.

Signed-off-by: Ronny Hegewald <Ronny.Hegewald@online.de>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>