Convert vm status strings to constants

This patch replaces all vmstatuses to constants.
Additionally added not documented 'Powering up' status to schema.

Change-Id: I95e6945c2ee9d443e22586f04dc0f9646cc5cbb3
Signed-off-by: Vinzenz Feenstra <vfeenstr@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/21790
Reviewed-by: Francesco Romani <fromani@redhat.com>
Tested-by: Francesco Romani <fromani@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>