include/qemu: rename Windows context definitions to expose bitness

Context structure in 64-bit Windows differs from 32-bit one and it
should be reflected in its name.

Signed-off-by: Viktor Prutyanov <viktor.prutyanov@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20220406171558.199263-2-viktor.prutyanov@redhat.com>