KVM: Use prepare/commit hooks to handle generic memslot metadata updates

Handle the generic memslot metadata, a.k.a. dirty bitmap, updates at the
same time that arch handles it's own metadata updates, i.e. at memslot
prepare and commit.  This will simplify converting @new to a dynamically
allocated object, and more closely aligns common KVM with architecture
code.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Reviewed-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
Message-Id: <2ddd5446e3706fe3c1e52e3df279f04c458be830.1638817640.git.maciej.szmigiero@oracle.com>