Allow to register a callback with fw_cfg_add_callback()

Allow to register a callback with fw_cfg_add_callback()
fw_cfg_add_callback() checks if key has FW_CFG_WRITE_CHANNEL bit set
after masking the key with FW_CFG_ENTRY_MASK.

But as FW_CFG_ENTRY_MASK is ~(FW_CFG_WRITE_CHANNEL | FW_CFG_ARCH_LOCAL),
the bit is never set and function exits.

This patch corrects this by checking the bit before masking the value.

Signed-by-off: Laurent Vivier <[email protected]>
Acked-by: Gleb Natapov <[email protected]>
git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@5978 c046a42c-6fe2-441c-8c8c-71466251a162