qemu: blockcopy: Fix conditions when virStorageSource should be initialized

qemu: blockcopy: Fix conditions when virStorageSource should be initialized

Commit 4b58fdf280a which enabled block copy also for network
destinations needed to limit when the 'mirror' storage source is
initialized in cases when we e.g. don't have an appropriate backend.

Limiting it just to virStorageFileSupportsCreate is too restrictive as
for example we can't precreate block devices and thus wouldn't
initialize the 'mirror' but since it's a local source we'd try to
examine it. This would fail since it wouldn't be initialized.

Fix it by introducing a more granular check whether certain operations
are supported and fix the check interlocks.

https://bugzilla.redhat.com/show_bug.cgi?id=1778058

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