Remove Support for VixDiskLib 5.1.0

VixDiskLib 5.1.0 has a known bug intermittently causing a
segfault during disk open operations when the vCenter Server
does not provide an SSL thumbprint for the target host.  The
error depends upon the memory content.  This segfault has been
encountered during testing so support for this version has
been removed.  Only this version has this flaw.

Add capability to easily add other unsupported versions along
with a concise reason to be printed to the log.
no longer reference rmagick
Merge pull request #310 from jerryk55/vddk_remove_5.1.0

Remove Support for VixDiskLib 5.1.0
Merge pull request #314 from kbrock/gemfile_rmagic_cleanup

no longer reference rmagick
Get new method text from new miq_ae_method.default_method_text.

No longer using old default method text with started/ended logging.