i386/pc_sysfw: Ensure sysfw flash configuration does not conflict with IGVM

i386/pc_sysfw: Ensure sysfw flash configuration does not conflict with IGVM

When using an IGVM file the configuration of the system firmware is
defined by IGVM directives contained in the file. In this case the user
should not configure any pflash devices.

This commit skips initialization of the ROM mode when pflash0 is not set
then checks to ensure no pflash devices have been configured when using
IGVM, exiting with an error message if this is not the case.

Signed-off-by: Roy Hopkins <roy.hopkins@randomman.co.uk>
Acked-by: Gerd Hoffman <kraxel@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Pankaj Gupta <pankaj.gupta@amd.com>
Reviewed-by: Ani Sinha <anisinha@redhat.com>
Link: https://lore.kernel.org/r/c6166cfe128933b04003a9288566b7affe170dfe.1751554099.git.roy.hopkins@randomman.co.uk
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>