qemu-config: load modules when instantiating option groups

Right now the SPICE module is special cased to be loaded when processing
of the -spice command line option.  However, the spice option group
can also be brought in via -readconfig, in which case the module is
not loaded.

Add a generic hook to load modules that provide a QemuOpts group,
and use it for the "spice" and "iscsi" groups.

Fixes: #194
Fixes: https://bugs.launchpad.net/qemu/+bug/1910696


Cc: qemu-stable@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>