configure: add libdaxctl support

Add a pair of configure options --{enable,disable}-libdaxctl to control
whether QEMU is compiled with libdaxctl [1]. Libdaxctl is a utility
library for managing the device dax subsystem.

QEMU uses mmap(2) to maps vNVDIMM backends and aligns the mapping
address to the page size (getpagesize(2)) by default. However, some
types of backends may require an alignment different than the page
size. The 'align' option is provided to memory-backend-file to allow
users to specify the proper alignment.

For device dax (e.g., /dev/dax0.0), the 'align' option needs to match
the alignment requirement of the device dax, which can be fetched
through the APIs of libdaxctl version 57 or up.

[1] Libdaxctl is a part of ndctl project.
The project's repository is: https://github.com/pmem/ndctl



For more information about libdaxctl APIs, you can refer to the
comments in source code of: pmem/ndctl/daxctl/lib/libdaxctl.c.

Reviewed-by: Joao Martins <joao.m.martins@oracle.com>
Signed-off-by: Jingqi Liu <jingqi.liu@intel.com>
Message-Id: <20200429085011.63752-4-jingqi.liu@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>