Re-enable hypervisor debug as part of opening 4.9

Re-enable hypervisor debug as part of opening 4.9

AFAICT following bacbf0cb7349 "build: convert debug to Kconfig"
hypervisor debug enablement is controlled here, rather than in
Config.mk.

The release checklist says that when branching, the new staging should
have debug enabled.  It seems to me that I should be changing this
here, therefore.

As additional evidence, I offer e1d1c68ea8a3 "xen: disable debug
build" which went in between 4.8.0 RC5 and RC6.  It does not explain
why this was done but it does STM that reverting that change is right.

CC: Andrew Cooper <andrew.cooper3@citrix.com>
CC: George Dunlap <George.Dunlap@eu.citrix.com>
CC: Jan Beulich <jbeulich@suse.com>
CC: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
CC: Stefano Stabellini <sstabellini@kernel.org>
CC: Tim Deegan <tim@xen.org>
CC: Wei Liu <wei.liu2@citrix.com>
Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>