tools/libfsimage: Add `xen' to .h names and principal .so name

tools/libfsimage: Add `xen' to .h names and principal .so name

`fsimage' is rather general.  And we do not expect this library to be
very useful out of tree because of its unstable ABI.

So add the word `xen'.  This will avoid naming conflicts with anyone
else's fsimage library.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>