Create compatibility wrapper for HVMMEM_access and the new XENMEM_access...

... flags and make all flags available via LibVMI.

This PR adds support to the upcoming Xen change from HVMMEM_access to the more generic XENMEM_access (http://xenbits.xen.org/gitweb/?p=xen.git;a=commit;h=fce5281c5a7f37eb0157a32626f7184e106aafe6). I also renamed the incorrectly named LibVMI flag X_ON_WRITE flag to RX2RW and included the thus far missing flag N2RWX in this PR.