Modernize watch file

Fixes #909743 https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=909743

uscan info: Looking at $base = https://download.qemu.org/ with
    $filepattern = qemu-(?:[-_]?(\d[\-+\.:\~\da-zA-Z]*))(?i)(?:\.(?:tar\.xz|tar\.bz2|tar\.gz|zip|tgz|tbz|txz)) found
    $newfile     = qemu-4.2.0.tar.xz
    $newversion  = 4.2
    $lastversion = 4.2
uscan info: Matching target for downloadurlmangle: https://download.qemu.org/qemu-4.2.0.tar.xz
uscan info: Upstream URL(+tag) to download is identified as    https://download.qemu.org/qemu-4.2.0.tar.xz
uscan info: Filename (filenamemangled) for downloaded file: qemu-4.2.0.tar.xz
uscan info: Newest version of qemu on remote site is 4.2, local version is 4.2
uscan info:    => Package is up to date for from
      https://download.qemu.org/qemu-4.2.0.tar.xz
uscan info: Scan finished