Clean up includes so that osdep.h is included first and headers
which it implies are not included manually.
This commit was created with scripts/clean-includes.
Backports commit f2ad72b30e214d1e3e41dba36f855354dfa81832 from qemu
qobject_to_qstring() crashes on null, which is a trap for the unwary.
Return null instead, and simplify a few callers.
Backports commit 7f0278435df1fa845b3bd9556942f89296d4246b from qemu
