domcaps: Remove function initializing domain caps as unsupported

domcaps: Remove function initializing domain caps as unsupported

Commit 5751a0b6b1968bb2354b2ac21cc5938b93009590 added a helper function
called virDomainCapsFeaturesInitUnsupported which initialized all domain
capability features as unsupported.

When adding a new feature this would initialize it as unsupported also
for hypervisor drivers which the original author possibly didn't intend
to modify. To prevent accidental wrong value being reported in such case
revert back to initializing individual features in the hypervisor
drivers themselves.

This is not a straight revert as additonal patches modified how we store
the capabilities.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Cole Robinson <crobinso@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>