kexec: some #include adjustments

kexec: some #include adjustments

In the context of working on x86's elf_core_save_regs() I noticed there
were far more source files getting rebuilt than I would have expected.
While the main offender looks to have been fixmap.h including kexec.h,
also drop use of elfcore.h from kexec.h.

While adjusting machine_kexec.c also replace use of guest_access.h by
domain_page.h.

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