tools/xenstore: drop support for running under SunOS

tools/xenstore: drop support for running under SunOS

Since several years now xenstored is no longer capable to run under
SunOS, as the needed libxengnttab interfaces are not available there.

Several attempts to let the SunOS maintainers address this situation
didn't change anything in this regard.

For those reasons drop SunOS support in xenstored by removing the SunOS
specific code.

Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Julien Grall <jgrall@amazon.com>