Check if nfs/glusterfs export is already mounted

_mount_nfs() do not check if the nfs_share is already mounted on
the same mnt path and pollutes logs with warnings about
'mount.nfs: xxx busy or already mounted'.

The patch will add the method is_mounted to libvirt/utils.py and
use the findmnt tool to find if the same network share on the same target
mount point is already mounted.
If 'findmnt' is not available for any reason (part of utils-linux),
the check will always return False.

Closes-Bug: #1280306
Change-Id: I1d7384ba2f1f1bb82bfbae96cca2772266d2583e
Merge "Check if nfs/glusterfs export is already mounted"