hw/i386/xen/xen-hvm: move x86-specific fields out of XenIOState

hw/i386/xen/xen-hvm: move x86-specific fields out of XenIOState

In preparation to moving most of xen-hvm code to an arch-neutral location, move:
- shared_vmport_page
- log_for_dirtybit
- dirty_bitmap
- suspend
- wakeup

out of XenIOState struct as these are only used on x86, especially the ones
related to dirty logging.
Updated XenIOState can be used for both aarch64 and x86.

Also, remove free_phys_offset as it was unused.

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