Introduce os-win32.c and move polling functions from vl.c

This introduces os-win32.c. It is meant to carry win32 specific
functions thata are not relevant for all of QEMU as well as win32
versions of various pieces like signal handling etc.

Move win32 polling handler helper functions from vl.c to os-win32.c

Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Acked-by: Juan Quintela <quintela@redhat.com>
Acked-by: Richard Henderson <rth@redhat.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>