vhost: replace ffsl with ctzl

vhost: replace ffsl with ctzl

Avoid using the GNU extesion ffsl which is not implemented in musl libc.

The atomic_xchg() means we know that vhost_log_chunk_t will never be
larger than the 'long' type, so ctzl() is always sufficient.

See also commit fbeadf50 (bitops: unify bitops_ffsl with the one in
host-utils.h, call it bitops_ctzl) on why ctzl should be used instead
of ffsl.

Signed-off-by: Natanael Copa <[email protected]>
Reviewed-by: Paolo Bonzini <[email protected]>
Signed-off-by: Peter Maydell <[email protected]>