block: Add new BDRV_O_INCOMING flag to notice incoming live migration

block: Add new BDRV_O_INCOMING flag to notice incoming live migration

From original patch with Patchwork-id: 31110 by
Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>

"Add a flag to indicate that incoming migration is pending and care needs
to be taken for data consistency.  Block drivers should not modify the
image file before incoming migration is complete since the migration
source host is still using the image file."

The rationale for not using bdrv->read_only is the following.

"Unfortunately this is not possible because too many other places in QEMU
test bdrv_is_read_only() and use it for their own evil purposes.  For
example, ide_init_drive() will error out because read-only harddisks are
not supported.  We're mixing guest and host side read-only concepts so
this simpler alternative does not work."

Signed-off-by: Benoit Canet <benoit.canet@gmail.com>
Reviewed-by: Stefan Hajnoczi <stefanha@gmail.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>