test/py: Rework test_efi_capsule to not use virt-make-fs

test/py: Rework test_efi_capsule to not use virt-make-fs

FIXME: Reword more

The problem with using "virt-make-fs" to make a filesystem image is that
it is extremely slow. Switch to using the fs_helper functions we have
instead from the filesystem tests as these can add files to images and
are significantly faster and still do not require root access.

The main change here is that our mount point directory has changed from
"test_efi_capsule" to "scratch" and so we need to update other functions
too. As the disk image that we get created doesn't have a GPT, invoke
sgdisk to do a conversion first.

Signed-off-by: Tom Rini <trini@konsulko.com>