Quote mount point when copying and resizing xfs (#782681)

Attempting to grow an already mounted xfs where the mount point
contained spaces failed like this:

    Grow /dev/sdb4 from 1.00 GiB to 1.50 GiB
    + calibrate /dev/sdb4
    + grow partition from 1.00 GiB to 1.50 GiB
    + grow file system to fill the partition
      + xfs_growfs /tmp/File System Label
        Usage: xfs_growfs [options] mountpoint
        ...

Apply the rule and quote all uses of mount points within command lines.
This is also applied to copying xfs file systems even though it was safe
because it only ever used GParted generated mount points.

Also for the xfs copy operation switch unmounting of partitions to
specify mount points instead of partitions.  This is just to be
consistent with how it is done in all the online file system resizing
code.

Bug 782681 - btrfs partitions mounted with whitespace cannot be resized