Convert the vbox driver to net model enum, which requires adding
enum values for Am79C970A, Am79C973, 82540EM, 82545EM, 82543GC. We
preserve the same casing that vbox historically used for these model
names.

Remove the now unused virDomainNetStrcaseeqModelString
Acked-by: NMichal Privoznik <mprivozn@redhat.com>
Signed-off-by: NCole Robinson <crobinso@redhat.com>