fix wrong path while calling pygrub and libxl-save-helper

fix wrong path while calling pygrub and libxl-save-helper

in current xen x86_64, the default libexec directory is /usr/lib/xen/bin,
while the private binder is /usr/lib64/xen/bin. but some commands(pygrub,
libxl-save-helper) located in private binder directory is called from
libexec directory which lead to the following error:
1, for pygrub bootloader:

libxl: debug: libxl_bootloader.c:429:bootloader_disk_attached_cb: /usr/lib/xen/bin/pygrub doesn't exist, falling back to config path

2, for libxl-save-helper:

libxl: cannot execute /usr/lib/xen/bin/libxl-save-helper: No such file or directory
libxl: error: libxl_utils.c:363:libxl_read_exactly: file/stream truncated reading ipc msg header from domain 3 save/restore helper stdout pipe
libxl: error: libxl_exec.c:118:libxl_report_child_exitstatus: domain 3 save/restore helper [10222] exited with error status 255

there are two ways to fix above error. the first one is make such command
store in the /usr/lib/xen/bin and /usr/lib64/xen/bin(symbol link to
previous), e.g. qemu-dm. The second way is using private binder dir
instead of libexec dir. e.g. xenconsole.
For these cases, the latter one is suitable.

Signed-off-by: Bamvor Jian Zhang <bjzhang@suse.com>
Committed-by: Ian Campbell <ian.campbell@citrix.com>