rbd: shift byte count as a 64-bit value

rbd: shift byte count as a 64-bit value

Otherwise, reads of more than 2GB fail.  Until commit
7bbca9e290a9c7c217b5a24fc6094e91e54bd05d, reads of 2^41
bytes succeeded at least theoretically.

In fact, pdiscard ought to receive a 64-bit integer as the
count for the same reason.

Reported by Coverity.

Fixes: 7bbca9e290a9c7c217b5a24fc6094e91e54bd05d
Cc: qemu-stable@nongnu.org
Cc: kwolf@redhat.com
Cc: eblake@redhat.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
(cherry picked from commit e948f663e9334249c394b88926addcdd3f9e35cd)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>