daemon: Verify ext2/3/4 filesystem name before passing to mke2fs (RHBZ#978302).

Add a utility function (fstype_is_extfs) to match ext2/3/4 filesystem
names.  This is used in a couple of places.

When passing the mke2fs -t parameter, verify that the request is for
an ext2/3/4 filesystem.  Previously we did not check this, and neither
did mke2fs when the -F flag was also used.