Pass cpu instead of cpu_family to the NSIS installer script.

That script checks for "x86_64" which is the cpu value,
while cpu_family is "x86".

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20201125191833.964753-1-sw@weilnetz.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>