hw/9pfs: Ensure an error is reported to user if 9pfs mount tag is too long If the 9pfs mount tag is longer than MAX_TAG_LEN bytes, rather than silently truncating the tag which will likely break the guest OS, report an immediate error and exit QEMU * hw/9pfs/virtio-9p-device.c: Report error & exit if mount tag is too long Signed-off-by: Daniel P. Berrange <berrange@redhat.com> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>