arm: Clean up fragile use of error_is_set() in realize() methods

Using error_is_set(ERRP) to find out whether a function failed is
either wrong, fragile, or unnecessarily opaque.  It's wrong when ERRP
may be null, because errors go undetected when it is.  It's fragile
when proving ERRP non-null involves a non-local argument.  Else, it's
unnecessarily opaque (see commit 84d18f0).

I guess the error_is_set(errp) in the DeviceClass realize() methods
are merely fragile right now, because I can't find a call chain that
passes a null errp argument.

Make the code more robust and more obviously correct: receive the
error in a local variable, then propagate it through the parameter.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Acked-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Andreas Färber <afaerber@suse.de>
MAINTAINERS: Document QOM

Invented by Anthony. Maintained through my qom-next tree lately.

Cc: Anthony Liguori <aliguori@amazon.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Acked-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Andreas Färber <afaerber@suse.de>