qemu: Use qemuBuildChrChardevCommand for vhost-user-gpu backend chardev

qemu: Use qemuBuildChrChardevCommand for vhost-user-gpu backend chardev

Now that the API for qemuBuildChrChardevCommand is sane enough, we can
use it to centralize formatting of '-chardev' generally.

The 'virDomainVideoDef' doesn't use 'virDomainChrSourceDef' internally so
we create it for this occasion manually.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>