mkdir() only takes path argument on mingw32:

  CC    i386-softmmu/vl.o
/src/qemu/vl.c: In function 'qmp_add_default':
/src/qemu/vl.c:3763: error: too many arguments to function 'mkdir'
/src/qemu/vl.c:3769: error: too many arguments to function 'mkdir'
Signed-off-by: NBlue Swirl <blauwirbel@gmail.com>