qemu-pr-helper: use new libmultipath API

libmultipath has recently changed its API.  The new API supports multi-threaded
clients better.  Unfortunately there is no backwards-compatibility, so we just
switch to the new one.  Running QEMU compiled with the new library on the old
library will likely crash, while doing the opposite will cause QEMU not to
start at all (because udev, get_multipath_config and put_multipath_config
are undefined).

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
