imsm: retrieve nvme serial from sysfs

imsm: retrieve nvme serial from sysfs

Don't rely on SCSI ioctl for reading NVMe serials - SCSI emulation for
NVMe devices can be disabled in the kernel config. Instead, try to get a
serial from /sys/block/nvme*/device/serial. If that fails for whatever
reason (i.e. no such attribute in old kernels) - fall back to the SCSI
method.

This also moves some SCSI-specific code from imsm_read_serial() to
scsi_get_serial().

Signed-off-by: Artur Paszkiewicz <artur.paszkiewicz@intel.com>
Reviewed-by: Tomasz Majchrzak <tomasz.majchrzak@intel.com>
Reviewed-by: Alexey Obitotskiy <aleksey.obitotskiy@intel.com>
Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>