migration/savevm: respect qemu_fclose() error code in save_snapshot()

migration/savevm: respect qemu_fclose() error code in save_snapshot()

qemu_fclose() could return error, f.e. if bdrv_co_flush() will return
the error.

This validation will become more important once we will start waiting of
asynchronous IO operations, started from bdrv_write_vmstate(), which are
coming soon.

Signed-off-by: Denis V. Lunev <[email protected]>
Reviewed-by: "Dr. David Alan Gilbert" <[email protected]>
Reviewed-by: Vladimir Sementsov-Ogievskiy <[email protected]>
Reviewed-by: Juan Quintela <[email protected]>
CC: Kevin Wolf <[email protected]>
CC: Max Reitz <[email protected]>
CC: Stefan Hajnoczi <[email protected]>
CC: Fam Zheng <[email protected]>
CC: Juan Quintela <[email protected]>
CC: Denis Plotnikov <[email protected]>
Signed-off-by: Juan Quintela <[email protected]>