snapshot: Add flag to enable creating checkpoints in live state

snapshot: Add flag to enable creating checkpoints in live state

The default behavior while creating external checkpoints is to pause the
guest while the memory state is captured. We want the users to sacrifice
space saving for creating the memory save image while the guest is live
to minimize downtime.

This patch adds a flag that causes the guest not to be paused before
taking the snapshot.
 *include/libvirt/libvirt.h.in:
    - add new paused reason: VIR_DOMAIN_PAUSED_SNAPSHOT
    - add new flag for taking snapshot: VIR_DOMAIN_SNAPSHOT_CREATE_LIVE
 *tools/virsh-domain-monitor.c:
    - add string representation for VIR_DOMAIN_PAUSED_SNAPSHOT
 *tools/virsh-snapshot.c:
    - add support for VIR_DOMAIN_SNAPSHOT_CREATE_LIVE
 *tools/virsh.pod:
    - add docs for --live option added to use
    VIR_DOMAIN_SNAPSHOT_CREATE_LIVE flag