qemu: Check for valid save image formats when loading driver config

qemu: Check for valid save image formats when loading driver config

Checking for valid 'foo_image_format' settings in qemu.conf is not done
until the settings are used. Move the checks to
virQEMUDriverConfigLoadSaveEntry, allowing to report incorrect format
settings at driver startup.

This change was made easier by also changing the corresponding fields
in the virQEMUDriverConfig to 'int', which is more in line with the
other fields that represent enumerated types.

Signed-off-by: Jim Fehlig <jfehlig@suse.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>