build: disable vbox on cygwin

build: disable vbox on cygwin

Cygwin cannot build the vbox driver yet:

  CC       vbox/libvirt_driver_vbox_impl_la-vbox_glue.lo
In file included from vbox/vbox_glue.c:27:0:
vblox/vbox_XPCOMCGlue.c:63:3: error: #error "Port me"
 # error "Port me"
   ^
In file included from vbox/vbox_XPCOMCGlue.c:45:0,
                 from vbox/vbox_glue.c:27:
vbox/vbox_XPCOMCGlue.c: In function 'tryLoadOne':
vbox/vbox_XPCOMCGlue.c:98:46: error: 'DYNLIB_NAME' undeclared (first use in this function)
         if (virAsprintf(&name, "%s/%s", dir, DYNLIB_NAME) < 0)
	                                      ^
./util/virstring.h:245:31: note: in definition of macro 'virAsprintf'
                         strp, __VA_ARGS__)
			       ^

Rather than trying to figure out how to get dynamic loading of
vbox to work under cygwin (since I don't even have a working vbox
setup to test whether it works), I'm going to be lazy and just
default to not even trying vbox on cygwin.