virsh: Allow extracting 'return' section of QMP command in 'qemu-monitor-command'

virsh: Allow extracting 'return' section of QMP command in 'qemu-monitor-command'

Simplify gathering the actual return value from a passed-through QMP
command when using 'qemu-monitor-command' by adding '--return-value'
switch which just extracts the 'return' section and alternatively
reports an error if the section is not present.

This simplifies gathering of some test data where the full reply would
need to be trimmed just for the actual return value.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>