x86: allow limiting the max C-state sub-state

x86: allow limiting the max C-state sub-state

Allow limiting the max C-state sub-state by appending to the max_cstate
command-line parameter. E.g. max_cstate=1,0
The limit only applies to the highest legal C-state. For example:
 max_cstate = 1, max_csubstate = 0 ==> C0, C1 okay, but not C1E
 max_cstate = 1, max_csubstate = 1 ==> C0, C1 and C1E okay, but not C2
 max_cstate = 2, max_csubstate = 0 ==> C0, C1, C1E, C2 okay, but not C3
 max_cstate = 2, max_csubstate = 1 ==> C0, C1, C1E, C2 okay, but not C3

Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>