conf: Make net model enum compare case insensitive

conf: Make net model enum compare case insensitive

vbox and vmx drivers do net case insensitive net model comparisons,
so for example 'VMXNET3' and 'vmxnet3' and 'VmxNeT3' in the XML will
translate to the same driver configuration. To convert these drivers
to use net model enum, we will need to do case insensitive comparisons
as well.

Essentially we implement virEnumToString, but with case insensitive
comparison. XML will always be formatted with the enum model string
we track internally, but we will accept any case insensitive variant.

Acked-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Cole Robinson <crobinso@redhat.com>