xen/cmdline: Work around some specific command line warnings

xen/cmdline: Work around some specific command line warnings

Xen will warn when an unknown parameter is found in the command line.  e.g.

  (d8) [ 1556.334664] (XEN) parameter "pv-shim" unknown!

One case where this goes wrong is a workaround for an old grub bug, which
resulted in "placeholder" being prepended to the command line.

Another case is when booting a CONFIG_PV_SHIM_EXCLUSIVE build, in which the
parsing for the "pv-shim" parameter is discarded.

Introduce ignore_param() and OPT_IGNORE to cope with known cases, where
issuing a warning is the wrong course of action to take.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Release-acked-by: Juergen Gross <jgross@suse.com>