KVM: Migration bandwidth and downtime control

Introduce 2 new hypervisor options, migration_bandwidth and migration_downtime
and implement KVM migration bandwidth and downtime control.

migration_bandwidth controls KVM's maximal bandwidth during migration, in
MiB/s. Default value is 32 MiB/s, same as KVM's internal default. This option
is a global hypervisor option.

migration_downtime sets the amount of time (in ms) a KVM instance is allowed to
freeze while copying memory pages. This is useful when migrating busy guests,
as KVM's internal default of 30ms is too low for the page-copying algorithm to
converge. This is a per-instance option, with a default of 30ms, same as KVM's
internal default.

Signed-off-by: Apollon Oikonomopoulos <apollon@noc.grnet.gr>
Signed-off-by: Balazs Lecz <leczb@google.com>
Reviewed-by: Balazs Lecz <leczb@google.com>