app-emulation/qemu: Add xdp USE flag

This will add an xdp USE flag to fix situations where users have the
bpf USE flag disabled but libxdp is installed thru net-libs/xdp-tools.
This results in QEMU automatically detecting this and attempting to
build af-xdp and failing.

Credit-to: holgerh, parona
Closes: https://bugs.gentoo.org/920459
Signed-off-by: Christopher Fore <csfore@posteo.net>
Signed-off-by: Sam James <sam@gentoo.org>

app-emulation/qemu/metadata.xml

@@ -60,6 +60,7 @@
 		<flag name="vte">Enable terminal support (<pkg>x11-libs/vte</pkg>) in the GTK+ interface</flag>
 		<flag name="xattr">Add support for getting and setting POSIX extended attributes, through
 		<pkg>sys-apps/attr</pkg>. Requisite for the virtfs backend.</flag>
+		<flag name="xdp">Enable support for XDP through <pkg>net-libs/xdp-tools</pkg></flag>
 		<flag name="xen">Enables support for Xen backends</flag>
 	</use>
 	<upstream>