hw/ide: Fix crash when plugging a piix3-ide device into the x-remote machine

hw/ide: Fix crash when plugging a piix3-ide device into the x-remote machine

QEMU currently crashes when the user tries to do something like:

 qemu-system-x86_64 -M x-remote -device piix3-ide

This happens because the "isabus" variable is not initialized with
the x-remote machine yet. Add a proper check for this condition
and propagate the error to the caller, so we can fail there gracefully.

Message-Id: <20210416125256.2039734-1-thuth@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>