x86/shadow: fix and improve sh_page_has_multiple_shadows()

x86/shadow: fix and improve sh_page_has_multiple_shadows()

While no caller currently invokes the function without first making sure
there is at least one shadow [1], we'd better eliminate UB here:
find_first_set_bit() requires input to be non-zero to return a well-
defined result.

Further, using find_first_set_bit() isn't very efficient in the first
place for the intended purpose.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
[1] The function has exactly two uses, and both are from OOS code, which
    is HVM-only. For HVM (but not for PV) sh_mfn_is_a_page_table(),
    guarding the call to sh_unsync(), guarantees at least one shadow.
    Hence even if sh_page_has_multiple_shadows() returned a bogus value
    when invoked for a PV domain, the subsequent is_hvm_vcpu() and
    oos_active checks (the former being redundant with the latter) will
    compensate. (Arguably that oos_active check should come first, for
    both clarity and efficiency reasons.)
master commit: 2896224a4e294652c33f487b603d20bd30955f21
master date: 2023-03-24 11:07:08 +0100