KVM: Introduce gfn_to_index() which returns the index for a given level

This patch cleans up the code and removes the "(void)level;" warning
suppressor.

Note that we can also use this for PT_PAGE_TABLE_LEVEL to treat every
level uniformly later.

Signed-off-by: Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Avi Kivity <avi@redhat.com>