xenserver: Bring up physical devices before configuring local port.

If DHCP is in use, then the physical devices have to be up before we
configure the local port, otherwise the DHCP request will never hit the
wire and we have no hope of getting an IP address.

This problem has been here for a long time, but it was masked until
commit c170afc1 "xenserver: Really take devices down in
interface-reconfigure." actually caused devices to go down and stay down.

Fixes bug #1809 "vswitch upgrade broke the xenserver".