kvm tools: Use pread() and pwrite() in disk-image.c

kvm tools: Use pread() and pwrite() in disk-image.c

This patch convert lseek() + read()/write() pairs with pread()/pwrite(). The
performance of running the following command

  dd if=/dev/vda of=/dev/null

in the guest does not show significant performance differences but should be a
net win because of the reduction in system calls.

Signed-off-by: Pekka Enberg <penberg@kernel.org>