block: protect path_has_protocol from filenames with colons

path_has_protocol will erroneously return "true" if the colon is part
of a filename.  These names are common with stable device names produced
by udev.  We cannot fully protect against this in case the filename
does not have a path component (e.g. if the current directory is
/dev/disk/by-path), but in the common case there will be a slash before
and path_has_protocol can easily detect that and return false.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
