tools: Implement xc_map_foreign_range(s) in terms of common helper

tools: Implement xc_map_foreign_range(s) in terms of common helper

Both Linux and FreeBSD already implemented these functions using
identical helpers based on xc_map_foreign_pages. Make one copy of
these common helpers and switch all OSes to use them, even those which
previously had a specific lower level implementation of this
functionality.

This is makes two fewer low level interfaces to think about.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>