xenapi: kernel_cmdline moved out from template

The kernel's cmdline was set during the initial devstack machine
installation. Thus, during second runs, the kernel's cmdline was not
updated. This patch extracts append_kernel_cmdline, and configures
domU's kernel cmdline every time. As some networking parameters are
passed through the kernel cmdline, this patch makes it possible to
change the network configuration, even if a cached devstack exists.

Related to blueprint xenapi-devstack-cleanup

Change-Id: I3b7175f4e83326c3e28825ac50625f6bd2a9a029
Merge "xenapi: kernel_cmdline moved out from template"