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_qfloat() and qobject_to_qint() crash on null, which is a
trap for the unwary. Return null instead, and simplify a few callers.
Backports commit fcf73f66a67f5e58c18216f8c8651e38cf4d90af from qemu
