virshtest: Adapt 'virsh-read-bufsiz' and 'virsh-read-non-seekable'

virshtest: Adapt 'virsh-read-bufsiz' and 'virsh-read-non-seekable'

Test both situations (reading from non-regular file and reading a file
larger than (arbitrary) buffer size) via 'virshtest'.

To feed the pipe we need to create a thread that does it, but otherwise
it's fairly straightforward.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>