qemu: Fix guest-sync response time in qga command

qemu: Fix guest-sync response time in qga command

The current implementation sets the guest-sync timeout to the
smaller value between the default value (QEMU_AGENT_WAIT_TIME)
and agent->timeout, without considering the timeout passed
via the qga command.

This patch enhances the guest-sync timeout logic to use the
minimum value among the default value, agent->timeout, and
the timeout passed via the qga command.

Resolves: https://gitlab.com/libvirt/libvirt/-/issues/590
Signed-off-by: ray <honglei.wang@smartx.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>