conf: don't use virDomainVirtType in struct field

Use of enum types for struct fields is generally avoided since it causes
warnings if the compiler assumes the enum is unsigned. For example

commit 8e2982b5767a25e5da6533c65bfdc648c95b3c69
Author: Cole Robinson <crobinso@redhat.com>
Date: Tue Jul 24 16:27:54 2018 -0400

conf: Clean up virDomainDefParseCaps

Introduced a line:

if ((def->virtType = virDomainVirtTypeFromString(virttype)) < 0) {

which causes a build failure with CLang

conf/domain_conf.c:19143:65: error: comparison of unsigned enum expression < 0 is always false [-Werror,-Wtautological-compare]

as the compiler is free to optimize away the "< 0" check due to the
assumption that the enum type is unsigned and always in range.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>