domain: conf: Don't validate VM ostype/arch at daemon startup

domain: conf: Don't validate VM ostype/arch at daemon startup

When parsing XML, we validate the passed ostype + arch combo against
the detected hypervisor capabilities. This has led to the following
problem:

- Define x86 qemu guest
- qemu is inadvertently removed from the host
- libvirtd is restarted. fails to parse VM config since arch is removed
- 'virsh list --all' is now empty, user is wondering where their VMs went

Add a new internal flag VIR_DOMAIN_DEF_PARSE_SKIP_OSTYPE_CHECKS. Use
it when loading VM and snapshot configs from disk.

https://bugzilla.redhat.com/show_bug.cgi?id=1043572
(cherry picked from commit f1a89a8b6d1a1097e41a171a13b1984b06e8ab3e)