qga: drop < Vista compatibility

Building QGA for XP seems possible so far: the dependency on
libqemuutil.a implies building qemu-thread-win32.c, which requires
Vista API since commit 12f8def0 (v2.9). But qemu-thread isn't being
used in QGA, the resulting binary may still work on XP.  XP is no
longer supported for the past 4.5y, it's time to drop support for it.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20181122110039.15972-5-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>