virtfs-proxy-helper: switch from libcap to libcap-ng

virtfs-proxy-helper is the only user of libcap; everyone else is using
the simpler libcap-ng API.  Switch and remove the configure code to
detect libcap.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
[groug: - drop remaining -lcap from Makefile
        - fix error message in configure]
Signed-off-by: Greg Kurz <groug@kaod.org>