libxl: drop support for Xen < 4.6

libxl: drop support for Xen < 4.6

Currently the libxl driver claims support for Xen >= 4.4, but
Xen 4.4 and 4.5 are no longer supported upstream. Let's increase
the minimum supported Xen version to 4.6 and change the defined
LIBXL_API_VERSION to 0x040500, which is the API version defined
when Xen 4.6 was released.

Since Xen 4.6 contains a pkgconfig file, drop the now unused code
that falls back to using LIBVIRT_CHECK_LIB in the absence of
pkgconfig file. In addition, bumping the LIBXL_API_VERSION
required adjusting the calls to libxl_set_vcpuaffinity to account
for the extra parameter in the 0x040500 version of the API.

Signed-off-by: Jim Fehlig <jfehlig@suse.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>