xenserver: Fix bugs related to using xe-switch-network-backend in spec file.

Commit daf2ebb (xenserver: Use xe-switch-network-stack in RPM spec
file.) changed the spec file to use xe-switch-network-backend instead of
directly modifying "/etc/xensource/network.conf".  It incorrectly
assumed that the command was in the search path.  It also didn't take
into account that the command will remove the "openvswitch" service with
chkconfig.  This commit fixes those errors.

Signed-off-by: Justin Pettit <jpettit@nicira.com>