travis.yml: Add CI for macOS

Build the tests on macOS and test TCG. HVF doesn't work in travis.

sieve tests pass but they might timeout in travis, they were left out
because of that.

Suggested-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Roman Bolshakov <r.bolshakov@yadro.com>
Message-Id: <20200901085056.33391-9-r.bolshakov@yadro.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>