Add quoting around parameter when calling external mke2fs (RHBZ#1084921).

If you tried to write the appliance to a path containing a space, it
would fail like this:

supermin: ext2: creating empty ext2 filesystem '/media/bigon/Little disk/.guestfs-1000/appliance.d.p4ovrvj6/root'
mke2fs: invalid blocks 'disk/.guestfs-1000/appliance.d.p4ovrvj6/root' on device '/media/bigon/Little'

This is because there was missing quoting around the parameter passed
to mke2fs.

Thanks: Laurent Bigonville
https://bugzilla.redhat.com/show_bug.cgi?id=1084921