block: simplify path_is_absolute

On Windows, all the logic is already in is_windows_drive and
is_windows_drive_prefix.  On POSIX, there is no need to look
out for colons.

The win32 code changes the behaviour in some cases, we could have
something like "d:foo.img". The old code would treat it as relative
path, the new one as absolute. Now the path is absolute, because to
go from c:/program files/blah to d:foo.img you cannot say c:/program
files/blah/d:foo.img.  You have to say d:foo.img.  But you could also
say it's relative because (I think, at least it was like that in DOS
15 years ago) d:foo.img is relative to the current path of drive D.
Considering how path_is_absolute is used by path_combine, I think it's
better to treat it as absolute.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block.c

@@ -210,21 +210,14 @@ static int path_has_protocol(const char *path)
 
 int path_is_absolute(const char *path)
 {
-    const char *p;
 #ifdef _WIN32
     /* specific case for names like: "\\.\d:" */
-    if (*path == '/' || *path == '\\')
+    if (is_windows_drive(path) || is_windows_drive_prefix(path)) {
         return 1;
-#endif
-    p = strchr(path, ':');
-    if (p)
-        p++;
-    else
-        p = path;
-#ifdef _WIN32
-    return (*p == '/' || *p == '\\');
+    }
+    return (*path == '/' || *path == '\\');
 #else
-    return (*p == '/');
+    return (*path == '/');
 #endif
 }