Partial writes can still happen in sendmsg and recvmsg, if a
signal is received in the middle of a write. To handle this,
retry the operation with a new offset/bytes pair.
Reviewed-by: Juan Quintela <quintela@redhat.com>
Reviewed-by: Orit Wassermann <owasserm@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>