qemu_migration: Use VIR_DOMAIN_PAUSED_API_ERROR

qemu_migration: Use VIR_DOMAIN_PAUSED_API_ERROR

Other APIs that internally use QEMU migration and need to temporarily
suspend a domain already report failure to resume vCPUs by setting
VIR_DOMAIN_PAUSED_API_ERROR state reason and emitting
VIR_DOMAIN_EVENT_SUSPENDED event with
VIR_DOMAIN_EVENT_SUSPENDED_API_ERROR.

Let's do the same in qemuMigrationSrcRestoreDomainState for consistent
behavior.

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