Thank you for the quick response.

> Strange way to do this, as libguestfs can do all this.

Sorry to ask this (I'm a newly graduated engineer and I new to the virtualization world), but how can libguestfs do all of the above?, do you mean it can create all the partitions inside a new lvm volume or a new raw image?? 

> Did you copy the syslinux.cfg file into the guest?  The error message
> you reported sounds like extlinux is installed but cannot read its
> config file.

Yes I'm sure that I copied the syslinux.cfg in the guest and that's why I was confused with the error message.
My problem is that the exact same steps  worked on a ubuntu 14.04  host with libguestfs 1.24.5 , and I can't see why it won't work on a centos7 host , that's why I thought about a problem with this libguestfs version (1.30.3).
Here's how I installed extlinux (I already saw the link that you sent me and I don't think I made a mistake) but the guest still don't want to boot:

[root@localhost libguestfs-1.30.3]# ./run guestfish -i -a /dev/vm_volumes/clone2

Welcome to guestfish, the guest filesystem shell for
editing virtual machine filesystems and disk images.

Type: 'help' for help on commands
      'man' to read the manual
      'quit' to quit the shell

Operating system: CentOS Linux release 7.1.1503 (Core) 
/dev/sda1 mounted on /

><fs> ls /boot/
.vmlinuz-3.10.0-229.el7.x86_64.hmac
System.map-3.10.0-229.el7.x86_64
config-3.10.0-229.el7.x86_64
grub
grub2
initramfs-0-rescue-c898899928d341b58ae4d02802d19340.img
initramfs-3.10.0-229.el7.x86_64.img
initramfs-3.10.0-229.el7.x86_64kdump.img
initrd-plymouth.img
ldlinux.sys
mbr.bin
symvers-3.10.0-229.el7.x86_64.gz
syslinux.cfg
vmlinuz-0-rescue-c898899928d341b58ae4d02802d19340
vmlinuz-3.10.0-229.el7.x86_64
><fs> cat /boot/syslinux.cfg 
DEFAULT linux
        LABEL linux
          SAY Booting the kernel
          KERNEL /boot/vmlinuz-3.10.0-229.el7.x86_64
          INITRD /boot/initramfs-3.10.0-229.el7.x86_64.img
          APPEND ro root=UUID=8e137cd9-cd6c-4205-a27f-211d6184a5f3

><fs> copy-file-to-device /boot/mbr.bin /dev/sda size:440
><fs> extlinux /boot
><fs> part-set-bootable /dev/sda 1 true

Thank you for your time and understanding.