qemu: Introduce and wire in 'VIR_MIGRATE_PARAM_MIGRATE_DISKS_DETECT_ZEROES'

qemu: Introduce and wire in 'VIR_MIGRATE_PARAM_MIGRATE_DISKS_DETECT_ZEROES'

The new 'VIR_MIGRATE_PARAM_MIGRATE_DISKS_DETECT_ZEROES' migration
parameter allows users of migration to pass in a list of disks where
zero-detection (which avoids transferring the zeroed-blocks) should be
enabled for the migration connection. This comes at the cost of extra
CPU cycles needed to check each block if it's all-zero.

This is useful for storage backends where information about the
allocation state of a block is not available and thus without this the
image would become fully allocated on the destination.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Pavel Hrdina <phrdina@redhat.com>