These patches aim to solve two types of DMA-reentrancy issues:
1.) mmio -> dma -> mmio case
To solve this, we track whether the device is engaged in io by
checking/setting a reentrancy-guard within APIs used for MMIO access.
2.) bh -> dma write -> mmio case
This case is trickier, since we dont have a generic way to associate a
bh with the underlying Device/DeviceState. Thus, this version allows a
device to associate a reentrancy-guard with a bh, when creating it.
(Instead of calling qemu_bh_new, you call qemu_bh_new_guarded)