ffsl is a GNU extension and not available in musl libc.

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 <ncopa@alpinelinux.org>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
[PMM: rebased to accommodate file rename to xen-hvm.c]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>