xen/Kconfig: define EXPERT a bool rather than a string

xen/Kconfig: define EXPERT a bool rather than a string

Since commit f80fe2b34f08 "xen: Update Kconfig to Linux v5.4" EXPERT
can only have two values (enabled or disabled). So switch from a string
to a bool.

Take the opportunity to replace all "EXPERT = y" to "EXPERT" and use
squash the lines bool and prompt together in modified place.

Signed-off-by: Julien Grall <jgrall@amazon.com>
Acked-by: Dario Faggioli <dfaggioli@suse.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>
Acked-by: Jan Beulich <jbeulich@suse.com>