hw/i386/xen: rearrange xen_hvm_init_pc

hw/i386/xen: rearrange xen_hvm_init_pc

In preparation to moving most of xen-hvm code to an arch-neutral location,
move non IOREQ references to:
- xen_get_vmport_regs_pfn
- xen_suspend_notifier
- xen_wakeup_notifier
- xen_ram_init

towards the end of the xen_hvm_init_pc() function.

This is done to keep the common ioreq functions in one place which will be
moved to new function in next patch in order to make it common to both x86 and
aarch64 machines.

Signed-off-by: Vikram Garhwal <vikram.garhwal@amd.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@amd.com>
Reviewed-by: Paul Durrant <paul@xen.org>