module: Don't complain when a module is absent

module: Don't complain when a module is absent

The current implementation depends on a configure-time generated list of
block modules. When any of them is absent, module_load() emits a warning.

This is suboptimal because extracting code to modules was mainly done to
allow separate packaging of modules with intrusive dependencies. Absence
of optional packages then leads to absence of modules and an error
message, which users may recognize as new and report as error.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Michael Tokarev <mjt@tls.msk.ru>
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>