vmnetx.generate: Explicitly pass input disk format to qemu-img

to prevent any misinterpretation of a raw disk as another format.
vmnetx.generate: Optionally write raw disk and uncompressed memory
vmnetx.domain: Optionally keep existing UUID in get_for_storage()

for cases in which the domain XML is rewritten more than once.
vmnetfs: Don't return -ENOSYS in poll handler

For some optional file operations like poll(), the FUSE kernel module
handles an -ENOSYS return by setting a filesystem-level flag that causes
all future attempts at that operation to be failed without querying
userspace first.  As a result, calling select()/poll() on an image file
would cause all future poll()s on a stat or stream file in the same FS to
return immediately.

When a file does not support poll(), unconditionally report it readable
and writable, as other filesystems do.