travis: Fix build with newer Qemu

Qemu requires a bleeding edge version of Python, not found in the current
travis environment.  Skip building Qemu in that case.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Wei Liu <wl@xen.org>

scripts/travis-build

@@ -16,6 +16,11 @@ cfgargs+=("--disable-rombios")
 cfgargs+=("--enable-docs")
 cfgargs+=("--with-system-seabios=/usr/share/seabios/bios.bin")
 
+# Qemu requires Python 3.5 or later
+if ! type python3 || python3 -c "import sys; res = sys.version_info < (3, 5); exit(not(res))"; then
+    cfgargs+=("--with-system-qemu=/bin/false")
+fi
+
 if [[ "${XEN_TARGET_ARCH}" == "x86_64" ]]; then
     cfgargs+=("--enable-tools")
 else