Fix ConnectionFactory auto-configuration with XA

Previously, the regular jms connection factory was always overriding
the xaConnectionFactory when using ActiveMQ. While
ActiveMQAutoConfiguration is namely shielded with a condition on a
missing ConnectionFactory bean, said configuration class also imports the
XA and regular configuration classes that both can create the connection
factory.

This commit adds a ConditionalOnMissingBean in the second class that is
imported in case the XA configuration has already defined what it needs.

Fixes gh-1727