commands/tpm: Skip loopback image measurement

commands/tpm: Skip loopback image measurement

The loopback image is configured to function as a disk by being mapped
as a block device. Instead of measuring the entire block device we
should focus on tracking the individual files accessed from it. For
example, we do not directly measure block devices like hd0 disk but the
files opened from it.

This method is important to avoid running out of memory since loopback
images can be very large. Trying to read and measure the whole image at
once could cause out of memory errors and disrupt the boot process.

Signed-off-by: Michael Chang <mchang@suse.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>