core: Check all attached VMs when updating shared disk boot flag

When updating a VM disk to be bootable, the other VMs that the disk is
attached to were checked to see if they already have a bootable disk
attached and if so the operation would have failed giving an error message
that the VM already has a bootable disk attached.
This check was changed to check all VMs and after iterating on all if one
or more contained a bootable disk fail and give an error message containing
all of the VMs that the operation failed on so the user will have all the
info in the first try
Also introduced a new DAO method to retrieve a VM bootable disk from the DB

Bug-Url: https://bugzilla.redhat.com/949923
Change-Id: I82ee07e02e08d60f559017d9f8205ab7df41c5c3
Signed-off-by: Tal Nisan <tnisan@redhat.com>