ruby: Fix .new method (RHBZ#1046509).

The .new method was unintentionally broken in
commit 9466060201600db47016133d80af22eb38091a49.

This fixes the .new method and allows it to be called with multiple
parameters, so you can use:

  Guestfs::Guestfs.new
  Guestfs::Guestfs.new()
  Guestfs::Guestfs.new(:close_on_exit => false)
  etc.

For backwards compatibility, Guestfs::create may still be used.

This commit also adds regression tests:

 - Use .new method in regular tests.  (Because this was not done
   before, we didn't catch the breakage.)

 - Test that ::create still works.

 - Test that args can be passed to .new method.