In the UK, an x at the end of a message is a kiss. Two xx is two kisses, and so on.
It's not an emoji, though. We've been putting x's at the end of our letters and notes for decades - long, long before the advent of emojis.
It's very common. I'd say that 90% of the messages I get from my friends and family have an x or several x's at the end. It's simply an affectionate sign-off, especially among women and girls.