Move to data-based hvparam checks instead of code

Currently the hypervisor parameters are checked using hard-coded snippets in
each hypervisor. However, most parameter checks fall into three cases:
  - file check
  - directory check
  - string value in a set

And the remaining ones are checked using simple functions.

This patch moves to a declarative-style for these parameter checks; in
hv_base we add the necessary infrastructure for these checks, and the
above common cases.

This translates into complete removal of the Check/Verify functions for
the Xen hypervisors, and a drastic reduction for the KVM one (which has
inter-parameter dependencies and thus can't use a simple table).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>