virdevmapper: Always use device name for finding targets

virdevmapper: Always use device name for finding targets

DM_TABLE_DEPS expects a device name in dm_ioctl.name. In one of the
cases, full path of the device was getting returned causing the ioctl
call to fail with `ENXIO (No such device or address)`.

Also rename the function and variable names to better reflect that we
are dealing with DM device names and not paths.

This got introduced in 22494556542c676d1b9e7f1c1f2ea13ac17e1e3e
Resolves: https://gitlab.com/libvirt/libvirt/-/issues/790

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Bhavin Gandhi <bhavin192@geeksocket.in>