If there are less than 2 arguments in version_ge(), the second
'shift' prints this error:
    ../configure: line 232: shift: shift count out of range

As Eric suggested, we can use 'shift ${2:+2}' which works out to
'shift 2' if $2 is set, or 'shift' (implicitly shift 1) if $2
is not set.

This patch replaces both 'shift; shift' occurrences in version_ge()
with 'shift ${2:+2}'.

Suggested-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <20200821203558.10338-1-sgarzare@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>