package/qemu: virtfs needs system

package/qemu: virtfs needs system

build of qemu with virtfs but without system fails since bump to version
6.0.0 in commit	6b86c9335fc3ff381878156c6243454d4b688df9:

../meson.build:1077:6: ERROR: Problem encountered: virtio-9p (virtfs) needs system emulation support

Indeed virtfs needs system support since switch to meson in
https://gitlab.com/qemu-project/qemu/-/commit/69202b406e3a42621a063c3afe80f580d343f59e

Fixes:
 - http://autobuild.buildroot.org/results/ee4b2bbc08c58dc972d285eafa43b7ad6586f270

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>