qemu: qemuOpenFileAs - set flag VIR_FILE_OPEN_FORCE_MODE

qemu: qemuOpenFileAs - set flag VIR_FILE_OPEN_FORCE_MODE

In the event we're falling into the code that tries to create the file
in a forked environment (VIR_FILE_OPEN_FORK) we pass different mode bits,
but those are never set because the virFileOpenForceOwnerMode has a check
if the OPEN_FORCE_MODE bit is set before attempting to change the mode.

Since this is a special case it seems reasonable to set u+rw,g+rw,o