Fix building LibVMI with xen-events on old Xen libraries and add static ...

...conversion matrix for flags.

Old xen libs don't define HVMMEM_access_n2rwx (for example the Ubuntu version Travis-CI has) thus failing to build with --enable-xen-events.
Does this need a range check of some sort?  I see that the existing `switch` statement does not have a `default` case, but this seems somehow more dangerous to me.