xen/pv-on-hvm:kexec: Fix implicit declaration of function 'xen_hvm_domain'

Randy found a compile error when using make randconfig to trigger

drivers/xen/xenbus/xenbus_xs.c:909:2: error: implicit declaration of function 'xen_hvm_domain'

it is unclear which of the CONFIG options triggered this. This
patch fixes the error.

Reported-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>