qemu: clean up MSI-X table handling

qemu: clean up MSI-X table handling

This patch does cleaning up of QEMU MSI handling. The fixes are:
1. Changes made to MSI-X table mapping handling to eliminate the small
windows in which guest could have access to physical MSI-X table.
2. MSI-X table is mapped as read-only to QEMU, as masking of MSI-X is
already in Xen now.
3. For registers that coexists inside the MSI-X table (this could be
only PBA I think), value read from physical page would be returned.

Signed-off-by:  Shan Haitao <maillists.shan@gmail.com>
Consolidated duplicate code into _pt_iomem_helper(). Fixed formatting.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Haitao Shan <haitao.shan@intel.com>
Acked-by: Stefano Stabellini <Stefano.Stabellini@eu.citrix.com>