Use sync mode in kparx when mapping partitions

Pass -s flag to kpartx to fix a potential race condition when deploying
(ext|ntfs)dump images. Without -s, kpartx will return right after
signaling the device mapper to create the partition block devices. This
does not guarantee that the partitions will have been created when
snf-image will try to use them.
Use iflag=fullblock in dd when copying the image

When copying the image data to the VM's hard disk, use iflag=fullblock
to accumulate full blocks of input. Without this flag, dd will write
the read data whenever read() syscall returns, without waiting to gather
enough data to create a bs-sized block. This may lead to unoptimized
performance.