qga: Refactor guest-exec capture-output to take enum

qga: Refactor guest-exec capture-output to take enum

Previously capture-output was an optional boolean flag that either
captured all output or captured none. While this is OK in most cases, it
lacks flexibility for more advanced capture cases, such as wanting to
only capture stdout.

This commits refactors guest-exec qapi to take an enum for capture mode
instead while preserving backwards compatibility.

Suggested-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
Signed-off-by: Konstantin Kostiuk <kkostiuk@redhat.com>