hv_balloon: fix printk loglevel

We have a mix of different ideas of which loglevel should be used. Unify
on the following:
- pr_info() for normal operation
- pr_warn() for 'strange' host behavior
- pr_err() for all errors.

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>