qemu-config: load modules when instantiating option groups

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>
(cherry picked from commit 632a8873500d27022c584256afc11e57e2418b94)
Signed-off-by: Michael Roth <michael.roth@amd.com>