meson: fix relpath failure on Win32

meson: fix relpath failure on Win32

On win32, os.path.relpath can raise an exception when computing
for example C:/msys64/mingw64/x.exe relative to E:/path/qemu-build.
Use try...except to avoid this, just using an absolute path in
this case.

Signed-off-by: Yonggang Luo <luoyonggang@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>