OvmfPkg/XenPlatformPei: Setup HyperPages earlier

OvmfPkg/XenPlatformPei: Setup HyperPages earlier

We are going to need to make an hypercall in order to retreive the E820
table from the hypervisor before been able to setup the memory.

Calling XenConnect earlier will allow to setup the XenHypercallLib
earlier to allow to make hypercalls.

While here, add some comments in XenConnect().

Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=1689
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20190813113119.14804-20-anthony.perard@citrix.com>