build-sys: add --enable-sanitizers

Typical slowdown introduced by AddressSanitizer is 2x.
UBSan shouldn't have much impact on runtime cost.

Enable it by default when --enable-debug, unless --disable-sanitizers.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20180116151152.4040-3-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>