qemu_capabilities: Introduce QEMU_CAPS_NETDEV_USER

qemu_capabilities: Introduce QEMU_CAPS_NETDEV_USER

Since -netdev user can be disabled during QEMU compilation, we
can't blindly expect it to just be there. We need a capability
that tracks its presence.

For qemu-4.2.0 we are not able to detect the capability so do the
next best thing - assume the capability is there. This is
consistent with our current behaviour where we blindly assume the
capability, anyway.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>