tpm: Clean up driver registration & lookup

We have a strict separation between enum TpmType and be_drivers[]:

* TpmType may have any number of members.  It just happens to have one.

* tpm_register_driver() uses the first empty slot in be_drivers[].

  If you register more than tpm_models[] has space,
  tpm_register_driver() fails.  Its caller silently ignores the
  failure.

  If you register more than one with a given TpmType,
  tpm_display_backend_drivers() will shows all of them, but
  tpm_driver_find_by_type() and tpm_get_backend_driver() will find
  only the one one that registered first.

Since we only ever register one driver, and be_drivers[] has space for
just that one, this contraption even works.

Turn be_drivers[] into a straight map from enum TpmType to driver.
Much simpler, and has a decent chance to actually work should we ever
acquire additional drivers.

While there, use qapi_enum_parse() in tpm_get_backend_driver().

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20170822132255.23945-8-marcandre.lureau@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
[Rebased, superfluous initializer dropped, commit message rewritten]
Cc: Stefan Berger <stefanb@us.ibm.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <1503564371-26090-4-git-send-email-armbru@redhat.com>
