qemu: Add optional unit to qemuMigrationParamsTPMapItem

qemu: Add optional unit to qemuMigrationParamsTPMapItem

Some migration parameters supported by libvirt may use units that differ
from the units used by QEMU for the corresponding parameters. For
example, libvirt defines migration bandwidth in MiB/s while QEMU expects
B/s. Let's add a unit field to qemuMigrationParamsTPMapItem for
automatic conversion when translating between libvirt's migration typed
parameters and QEMU's migration paramteres.

This patch is a preparation for future parameters as the existing
VIR_MIGRATE_PARAM_BANDWIDTH parameter is set using "migrate_set_speed"
QMP command rather than "migrate-set-parameters" for backward
compatibility.

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