qemu: Make qemuDomainCheckDiskStartupPolicy self-contained
qemu: Make qemuDomainCheckDiskStartupPolicy self-contained

There is an error reset following the function and check for
startupPolicy before that.  Let's reflect those things inside that
function so that future code doesn't have to be that complex.

Signed-off-by: Martin Kletzander <e82f23e74d43760279ebcc505dac4ae70d5627d0@redhat.com>

andreabolognani/libvirt,crobinso/libvirt,jardasgit/libvirt,datto/libvirt,zippy2/libvirt,jfehlig/libvirt,andreabolognani/libvirt,datto/libvirt,eskultety/libvirt,taget/libvirt,fabianfreyer/libvirt,crobinso/libvirt,eskultety/libvirt,eskultety/libvirt,olafhering/libvirt,VenkatDatta/libvirt,VenkatDatta/libvirt,fabianfreyer/libvirt,jardasgit/libvirt,jardasgit/libvirt,taget/libvirt,eskultety/libvirt,taget/libvirt,libvirt/libvirt,olafhering/libvirt,andreabolognani/libvirt,zippy2/libvirt,eskultety/libvirt,datto/libvirt,fabianfreyer/libvirt,VenkatDatta/libvirt,taget/libvirt,zippy2/libvirt,nertpinx/libvirt,taget/libvirt,VenkatDatta/libvirt,datto/libvirt,datto/libvirt,libvirt/libvirt,andreabolognani/libvirt,nertpinx/libvirt,libvirt/libvirt,crobinso/libvirt,jardasgit/libvirt,nertpinx/libvirt,VenkatDatta/libvirt,libvirt/libvirt,jfehlig/libvirt,jfehlig/libvirt,nertpinx/libvirt,nertpinx/libvirt,crobinso/libvirt,fabianfreyer/libvirt,olafhering/libvirt,olafhering/libvirt,fabianfreyer/libvirt,zippy2/libvirt,andreabolognani/libvirt,jardasgit/libvirt,jfehlig/libvirt