Export more instance parameters in instance export

Currently the backend parameters are not exported automatically, but
only a few directly in the '[instance]' section. Hypervisor type and
hypervisor parameters are not exported at all.

This patch creates two separate sections for the be and hv parameters,
and stores the parameters (including ones that come from the cluster
defaults, but not the hypervisor globals for example) in the export.
The import code is not changed yet.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>