Public API for post-copy migration bandwidth

Public API for post-copy migration bandwidth

This patch adds a new VIR_MIGRATE_PARAM_BANDWIDTH_POSTCOPY typed
parameter for virDomainMigrate3 and virDomainMigrateToURI3 for setting
maximum post-copy migration bandwidth.

In case the initial VIR_MIGRATE_PARAM_BANDWIDTH_POSTCOPY value turns out
to be suboptimal a new VIR_DOMAIN_MIGRATE_MAX_SPEED_POSTCOPY flag for
virDomainMigrateSetMaxSpeed and virDomainMigrateGetMaxSpeed may be used
to set/get the maximum post-copy migration bandwidth while migration is
already running.

Signed-off-by: Jiri Denemark <jdenemar@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>