configure: Enable werror for git worktrees

configure: Enable werror for git worktrees

The configure script checks multiple times whether it works in a git
repository and it does this by "test -e "${source_path}/.git" in 4 cases
but in one case where it tries to enable werror "-d" is used there which
fails on git worktrees as .git is a file then and not a directory.

This changes the test to "-e" as other occurrences.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Message-Id: <20190228043503.68494-1-aik@ozlabs.ru>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>