qemu: avoid memory leaks

qemu: avoid memory leaks

Quite a few leaks detected by coverity.  For chr, the leaks were
close enough to the allocations to plug in place; for disk, the
leaks were separated from the allocation by enough other lines with
intermediate failure cases that I refactored the cleanup instead.

* src/qemu/qemu_command.c (qemuParseCommandLine): Plug leaks.