-machine vmport=auto: Fix handling of VMWare ioport emulation for xen

c/s 9b23cfb76b3a5e9eb5cc899eaf2f46bc46d33ba4

or

c/s b154537ad07598377ebf98252fb7d2aff127983b

moved the testing of xen_enabled() from pc_init1() to
pc_machine_initfn().

xen_enabled() does not return the correct value in
pc_machine_initfn().

Changed vmport from a bool to an enum.  Added the value "auto" to do
the old way.  Move check of xen_enabled() back to pc_init1().

Acked-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Don Slutz <dslutz@verizon.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
