virtio-gpu: Optimize 2D resource data transfer

The following points sometimes can reduce much data
to copy:
1. When width matches, we can transfer data with one
call of iov_to_buf().
2. Only the required height need to transfer, not
whole image.

Signed-off-by: Keqian Zhu <zhukeqian1@huawei.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20230612021358.25068-1-zhukeqian1@huawei.com>