cmd: qfw: make fwcfg_present and fwcfg_dma_present public

This patch is part of the qfw refactor work. This patch makes
qemu_fwcfg_present() and qemu_fwcfg_dma_present() public functions.

Signed-off-by: Miao Yan <yanmiaobest@gmail.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>