qemu: migration: Remove QEMU_MONITOR_MIGRATE_BACKGROUND

qemu: migration: Remove QEMU_MONITOR_MIGRATE_BACKGROUND

'qemuMonitorJSONMigrate' is called from:
 - qemuMonitorMigrateToHost
 - qemuMonitorMigrateToSocket
   Both of the above function are called only from
   qemuMigrationSrcStart.

 - qemuMonitorMigrateToFd
   - called from:
     - qemuMigrationSrcToFile
       Both instances here pass QEMU_MONITOR_MIGRATE_BACKGROUND
       directly.
     - qemuMigrationSrcStart

qemuMigrationSrcStart is then called from qemuMigrationSrcRun and
qemuMigrationSrcResume, both of which always add QEMU_MONITOR_MIGRATE_BACKGROUND
to the flags.

Thus any caller always passes the flag so that we can remove the flag
altogether.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>