vhost: add dynamic logging system

vhost: add dynamic logging system

Currently there are a couple of limitations on the logging system: Most
of the logs are compiled out and both datapath and controlpath logs
share the same loglevel.

This patch tries to help fix that situation by:
- Splitting control plane and data plane logs
- Making control plane logs dynamic while keeping data plane logs
  compiled out by default for log levels lower than the INFO.

As a result, two macros are introduced:
- VHOST_LOG_CONFIG(LEVEL, ...): Config path logging. Level can be
  dynamically controlled by "lib.vhost.config"

- VHOST_LOG_DATA(LEVEL, ...): Data path logging. Level can be
  dynamically controlled by "lib.vhost.data". Every log macro with a
  level lower than RTE_LOG_DP_LEVEL (which defaults to RTE_LOG_INFO)
  will be compiled out.

Signed-off-by: Adrian Moreno <amorenoz@redhat.com>
Acked-by: Tiwei Bie <tiwei.bie@intel.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>