A cygwin build of the qemu driver fails with:

qemu/qemu_process.c: In function 'qemuPrepareCpumap':
qemu/qemu_process.c:1803:31: error: 'CPU_SETSIZE' undeclared (first use in this function)

CPU_SETSIZE is a Linux extension in <sched.h>; a bit more portable
is using sysconf if _SC_NPROCESSORS_CONF is defined (several platforms
have it, including Cygwin).  Ultimately, I would have preferred to
use gnulib's 'nproc' module, but it is currently under an incompatible
license.

* src/qemu/qemu_conf.h (QEMUD_CPUMASK_LEN): Provide definition on
cygwin.
Signed-off-by: NEric Blake <eblake@redhat.com>