The QGA MSI target requires several macros which are only available
without --disable-guest-agent-msi.

Don't define that target if configure was called with --disable-guest-agent-msi.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Message-Id: <20201117201834.408892-1-sw@weilnetz.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>