qtest: make QEMU our direct child process

qtest_init() cannot use exec*p() to launch QEMU since the exec*p()
functions take an argument array while qtest_init() takes char
*extra_args.  Therefore we execute /bin/sh -c <command-line> and let the
shell parse the argument string.

This left /bin/sh as our child process and our child's child was QEMU.
We still want QEMU's pid so the -pidfile option was used to let QEMU
report its pid.

The pidfile needs to be unlinked when the test case exits or fails.  In
other words, the pidfile creates a new problem for us!

Simplify all this using the shell 'exec' command.  It allows us to
replace the /bin/sh process with QEMU.  Then we no longer need to use
-pidfile because we already know our fork child's pid.

Note: Yes, it seems silly to exec /bin/sh when we could just exec QEMU
directly.  But remember qtest_init() takes a single char *extra_args
command-line fragment instead of a real argv[] array, so we need
/bin/sh's argument parsing behavior.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>

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