ci: Enable msys2 ci in cirrus

ci: Enable msys2 ci in cirrus

Install msys2 in a proper way refer to:
 https://github.com/cirruslabs/cirrus-ci-docs/issues/699
The https://wiki.qemu.org/Hosts/W32#Native_builds_with_MSYS2 need to be updated.
There is no need of --cross-prefix, open mingw64.exe instead of msys2.exe then
we don't need the --cross-prefix, besides we use environment variable settings:
    MSYS: winsymlinks:nativestrict
    MSYSTEM: MINGW64
    CHERE_INVOKING: 1
to opening mingw64 native shell.

Signed-off-by: Yonggang Luo <luoyonggang@gmail.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20200915121318.247-25-luoyonggang@gmail.com>
[thuth: Add --target-list-exclude to speed it up a little bit, and
        add capstone package to avoid build failure with internal capstone]
Signed-off-by: Thomas Huth <thuth@redhat.com>