Allow suspend during live migration

Allow suspend during live migration

Currently no command can be sent to a qemu process while another job is
active. This patch adds support for signaling long-running jobs (such as
migration) so that other threads may request predefined operations to be
done during such jobs. Two signals are defined so far:
    - QEMU_JOB_SIGNAL_CANCEL
    - QEMU_JOB_SIGNAL_SUSPEND

The first one is used by qemuDomainAbortJob.

The second one is used by qemudDomainSuspend for suspending a domain
during migration, which allows for changing live migration into offline
migration. However, there is a small issue in the way qemudDomainSuspend
is currently implemented for migrating domains. The API calls returns
immediately after signaling migration job which means it is asynchronous
in this specific case.

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>