build-sys: restrict vmcoreinfo to fw_cfg+dma capable targets

vmcoreinfo is built for all targets. However, it requires fw_cfg with
DMA operations support (write operation). Restrict vmcoreinfo exposure
to architectures that are supporting FW_CFG_DMA, that is arm-virt and
x86 only atm.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Daniel Henrique Barboza <danielhb@linux.vnet.ibm.com>
Tested-by: Daniel Henrique Barboza <danielhb@linux.vnet.ibm.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
