libelf: allow having HYPERCALL_PAGE entry before VIRT_BASE in __xen_guest section

libelf: allow having HYPERCALL_PAGE entry before VIRT_BASE in __xen_guest section

When filling __xen_guest section of a guest, user may define
HYPERCALL_PAGE earlier than VIRT_BASE in the section leading to an
incorrect hypercall page address since an undefined virt_base could be
used to compute hypercall page address.
If there is no VIRT_BASE entry in __xen_guest section, default value of
0 is used for virt_base. Thus, setting hypercall page address to
HYPERCALL_PAGE value is correct in this case too.

Signed-off-by: Gregory Herrero <gregory.herrero@oracle.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>