Make qemu_peek_buffer loop until it gets it's data

Make qemu_peek_buffer repeatedly call fill_buffer until it gets
all the data it requires, or until there is an error.

  At the moment, qemu_peek_buffer will try one qemu_fill_buffer if there
  isn't enough data waiting, however the kernel is entitled to return
  just a few bytes, and still leave qemu_peek_buffer with less bytes
  than it needed.  I've seen this fail in a dev world, and I think it
  could theoretically fail in the peeking of the subsection headers in
  the current world.

Comment qemu_peek_byte to point out it's not guaranteed to work for
  non-continuous peeks

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: ChenLiang <chenliang0016@icloud.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>
