XenServer: Add knowledge of vswitch to xen-bugtool

XenServer: Add knowledge of vswitch to xen-bugtool

The xen-bugtool tool gathers information that may be useful in debugging
problems.  This commit modifies that script to add support for vswitch.
Currently, the information we collect is:

    - /etc/ovs-vswitchd.conf
    - ovs-dpctl show
    - Cores in /var/xen/vswitch
    - ovs-ofctl show <bridges>
    - ovs-ofctl status <bridges>
    - ovs-ofctl dump-flows <bridges>
    - ovs-dpctl dump-flows <bridges>

This commit also modifies the way cores are handled.  They are now
enabled by default and placed in "/var/xen/vswitch".

Feature #1570