xen-hvm.c: Add support for Xen access to vmport

This adds synchronisation of the 6 vcpu registers (only 32bits of
them) that vmport.c needs between Xen and QEMU.

This is to avoid a 2nd and 3rd exchange between QEMU and Xen to
fetch and put these 6 vcpu registers used by the code in vmport.c
and vmmouse.c

The registers are passed in the new shared page provided by
HVM_PARAM_VMPORT_REGS_PFN.

Add new array to XenIOState that allows selection of current_cpu by
vcpu id.

Now pass XenIOState to handle_ioreq().

Add new routines regs_to_cpu(), regs_from_cpu(), and
handle_vmport_ioreq().

Signed-off-by: Don Slutz <dslutz@verizon.com>
Reviewed-by: Paul Durrant <paul.durrant@citrix.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>