qemuNodeDeviceDetachFlags: Avoid use of uninitialized variables

qemuNodeDeviceDetachFlags: Avoid use of uninitialized variables

After abf75aea24 the compiler screams:

qemu/qemu_driver.c: In function 'qemuNodeDeviceDetachFlags':
qemu/qemu_driver.c:10693:9: error: 'domain' may be used uninitialized in this function [-Werror=maybe-uninitialized]
     pci = virPCIDeviceNew(domain, bus, slot, function);
         ^
qemu/qemu_driver.c:10693:9: error: 'bus' may be used uninitialized in this function [-Werror=maybe-uninitialized]
qemu/qemu_driver.c:10693:9: error: 'slot' may be used uninitialized in this function [-Werror=maybe-uninitialized]
qemu/qemu_driver.c:10693:9: error: 'function' may be used uninitialized in this function [-Werror=maybe-uninitialized]

Since the other functions qemuNodeDeviceReAttach and qemuNodeDeviceReset
looks exactly the same, I've initialized the variables there as well.
However, I am still wondering why those functions don't matter to gcc
while the first one does.