virDomainDefCopy: Skip ostype checks

When parsing domain XML the virCapsDomainData lookup is performed
in order to fill in missing def->os.arch and def->os.machine
strings. Well, when doing copy of already existing virDomainDef
we don't want any automagic fill in of defaults (and those two
strings are going to be provided at this point anyway by first
parse of the domain XML).

What is even worse is that we do not look up capabilities for
parsed emulator path rather some generic capabilities for parsed
arch. Therefore, if emulator points to qemu under non-default
path (say $HOME/qemu-system-arm) but there's no such qemu under
the default path (say /usr/bin/qemu-system-arm) the capabilities
lookup fails and creating the copy is denied.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>