vswitch: Improve add_phy_port error messages

vswitch: Improve add_phy_port error messages

In case that vSwitch can't add required number of physical
ports, then error message should be printed. Previous implementation
was not consistent. Thus error handling has been united across
all vswitches and message explicitly states the cause of the failure.

NOTE: A better solution would be a refactoring of all vSwitch
classes to follow VPP port handling and to move port related
structures and physical port check into parent vswitch class.

JIRA: VSPERF-555

Change-Id: Id09a61432ea93e261f563254829348ac61f5dc8a
Signed-off-by: Martin Klozik <martinx.klozik@intel.com>
Reviewed-by: Jose Lausuch <jalausuch@suse.com>
Reviewed-by: Al Morton <acmorton@att.com>
Reviewed-by: Christian Trautman <ctrautma@redhat.com>
Reviewed-by: Sridhar Rao <sridhar.rao@spirent.com>
Reviewed-by: Trevor Cooper <trevor.cooper@intel.com>