hw/pci-host/prep: Do not use hw_error() in realize function

hw_error() is for CPU related errors only (it prints out a
register dump and calls abort()), so we should not use it
if we just failed to load the bios image. Apart from that,
realize() functions should not exit directly but always set
the errp with error_setg() in case of errors instead.
Additionally, move some code around and delete the bios memory
subregion again in case of such an error, so that we leave a
clean state when returning to the caller.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Hervé Poussineau <hpoussin@reactos.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>