Fix mingw32 and OpenBSD warnings

ffsl() is not universally available, so there are these warnings
on both mingw32 and OpenBSD:
/src/qemu/hw/pcie_aer.c: In function 'pcie_aer_update_log':
/src/qemu/hw/pcie_aer.c:399: warning: implicit declaration of function 'ffsl'

Since status field in PCIEAERErr is uint32_t, we can just use ffs() instead.

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>