oslib-posix: Print errors before aborting on qemu_alloc_stack()

oslib-posix: Print errors before aborting on qemu_alloc_stack()

If QEMU is running on a system that's out of memory and mmap()
fails, QEMU aborts with no error message at all, making it hard
to debug the reason for the failure.

Add perror() calls that will print error information before
aborting.

Signed-off-by: Eduardo Habkost <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Tested-by: Philippe Mathieu-Daudé <[email protected]>
Message-id: 20170829212053[email protected]
Signed-off-by: Stefan Hajnoczi <[email protected]>