grub: search for OEM by partition label instead of filesystem label

Probing all filesystem types on all block devices appears to hang
booting Amazon EC2 HVM instances. The console output is unreliably
buffered so there is no information on what the failure actually is. On
the up side we can work around it easily by only searching the GPT which
appears to be safe.
Merge pull request #361 from marineam/grub

grub: search for OEM by partition label instead of filesystem label