block: do not allocate an iovec per read of a growable/zero_after_eof BDS Most reads do not go past the end of the file, and they can use the input QEMUIOVector instead of creating one. This removes the qemu_iovec_* functions from the profile. Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Kevin Wolf <kwolf@redhat.com>