qemu: simplify list cleanup

qemu: simplify list cleanup

No need to open code now that we have a nice function.

Interestingly, our virStringFreeList function is typed correctly
(a malloc'd list of malloc'd strings is NOT const, whether at the
point where it is created, or at the point where it is cleand up),
so using it with a 'const char **' argument would require a cast
to keep the compiler.  I chose instead to remove const from code
even where we don't modify the argument, just to avoid the need
to cast.

* src/qemu/qemu_command.h (qemuParseCommandLine): Drop declaration.
* src/qemu/qemu_command.c (qemuParseProcFileStrings)
(qemuStringToArgvEnv): Don't force malloc'd result to be const.
(qemuParseCommandLinePid, qemuParseCommandLineString): Simplify
cleanup.
(qemuParseCommandLine, qemuFindEnv): Drop const-correctness to
avoid the need to cast in callers.

Signed-off-by: Eric Blake <eblake@redhat.com>