tools/libfsimage: Add `XEN' to environment variable name

tools/libfsimage: Add `XEN' to environment variable name

This library, which is private to Xen and was properly namespaced in
  1a814711881beb17f073f5f57e27e5bd4da1b956
  tools/libfsimage: Add `xen' to .h names and principal .so name
honours an environment variable to override the directory where
shared objects (ie filesystem plugins) are to be loaded from.

Rename that variable from FSIMAGE_FSDIR to XEN_FSIMAGE_FSDIR, to give
it a proper namespace prefix.

Nothing in xen.git sets this variable.  The three hits for the string
`FSIMAGE_FSDIR' are this getenv, and two references to a compile-time
manifest constant which provides the default value (the -D which sets
it, and the place it is used).

I have also checked the current Debian Xen package in buster and the
variable is not set there either.

CC: Andrew Cooper <andrew.cooper3@citrix.com>
CC: Jan Beulich <JBeulich@suse.com>
CC: George Dunlap <george.dunlap@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Release-acked-by: Juergen Gross <jgross@suse.com>