Fix boot failures.

Somebody decided that it was an awesome idea to turn 64-bit extents on by
default for mkfs.ext4. PV-GRUB apparently can't boot off of an ext4 volume that
has the 64-bit extents option (i.e. `64bit`) enabled, thus all hell broke loose
when our t1.micro configuration server did a `yum update`.

Explicitly disable 64-bit extents in build-ebs-image. Then, add an optional
`direct` method of copying the Xen image to EBS. The `direct` method does a
bit-for-bit copy of the Xen image to an EBS volume - rather than mount the Xen
image via the loopback driver, create another ext4 filesystem, and copy the
original Xen image's contents to it.
Use a minimal set of ext4 options.

GRUB doesn't support some of the more exciting features of ext4.
Use only those options required to have the volume qualify as ext4
(journal, extents). Add large file support too, just in case.