x86/boot: add cmdline to struct boot_domain

x86/boot: add cmdline to struct boot_domain

Add a container for the "cooked" command line for a domain. This
provides for the backing memory to be directly associated with the
domain being constructed.  This is done in anticipation that the domain
construction path may need to be invoked multiple times, thus ensuring
each instance had a distinct memory allocation.

Signed-off-by: Daniel P. Smith <dpsmith@apertussolutions.com>
Signed-off-by: Jason Andryuk <jason.andryuk@amd.com>
Signed-off-by: Alejandro Vallejo <agarciav@amd.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Denis Mukhin <dmukhin@ford.com>