hw/i386: turn off vmport if CONFIG_VMPORT is disabled

vmport device is not included when CONFIG_VMPORT is disabled, hence
QEMU fails with the following error:

`Unknown device 'vmport' for bus 'ISA': unknown.`

v2: imply VMPORT (Paolo Bonzini )

Signed-off-by: Julio Montes <julio.montes@intel.com>
Message-Id: <20190712160257.18270-1-julio.montes@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>