Revert "qemu_conf: Track embed root dir"

Revert "qemu_conf: Track embed root dir"

This reverts commit 06a19921b6a522cd7b4d352c9320909752947de3.

What I haven't realized when writing this ^^ commit is that the
virQEMUDriver structure already stores the root directory path.
And since the pointer is immutable it can be accessed right from
the structure and thus there is no need to duplicate it in the
driver config.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>