qdev: print error message before aborting

qdev: print error message before aborting

qdev_prop_set_* functions are always called by machine init functions
that should know what they're doing, so they abort on error.  Still,
an assert(!errp) does not aid debugging.  Print an error before aborting.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>