tests/boot-sector: Use mkstemp() to create a unique file name

The pxe-test is run for three different targets now (x86_64, i386
and ppc64), and the bios-tables-test is run for two targets (x86_64
and i386). But each of the tests is using an invariant name for the
disk image with the boot sector code - so if the tests are running in
parallel, there is a race condition that they destroy the disk image
of a parallel test program. Let's use mkstemp() to create unique
temporary files here instead - and since mkstemp() is returning an
integer file descriptor instead of a FILE pointer, we also switch
the fwrite() and fclose() to write() and close() instead.

Reported-by: Sascha Silbe <x-qemu@se-silbe.de>
Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
