tools/libxc: free thread specific hypercall buffer on xc_interface_close

tools/libxc: free thread specific hypercall buffer on xc_interface_close

The per-thread hypercall buffer is usually cleaned up on pthread_exit
by the destructor passed to pthread_key_create. However if the calling
application is not threaded then the destructor is never called.

This frees the data for the current thread only but that is OK since
any other threads will be cleaned up by the destructor.

Changed since v1:
 * Ensure hcall_buf_pkey is initialised before use. Thanks to
   Christoph Egger for his help diagnosing this issue on NetBSD.
 * Remove redundant if (hcall_buf) from xc_clean_hcall_buf since
   _xc_clean_hcall_buf includes the same check.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>