OvmfPkg/XenPlatformPei: Detect OVMF_INFO from hvmloader

OvmfPkg/XenPlatformPei: Detect OVMF_INFO from hvmloader

EFI_XEN_OVMF_INFO is only useful to retrieve the E820 table. The
mXenHvmloaderInfo isn't used yet, but will be use in a further patch to
retrieve the E820 table.

Also remove the unused pointer from the XenInfo HOB as that information
is only useful in the XenPlatformPei.

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: <20190813113119.14804-11-anthony.perard@citrix.com>