Libcap was dropped from virtio-9p (commit 7e462613), so remove it from
the dockerfiles as well.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Acked-by: Alex Bennée <alex.bennee@linaro.org>
[groug, mention SHA1 that dropped libcap]
Signed-off-by: Greg Kurz <groug@kaod.org>