OvmfPkg/XenPlatformPei: no hvmloader: get the E820 table via hypercall

OvmfPkg/XenPlatformPei: no hvmloader: get the E820 table via hypercall

When the Xen PVH entry point has been used, hvmloader hasn't run and
hasn't prepared an E820 table. The only way left to get an E820 table
is to ask Xen via an hypercall.  We keep the result cached to avoid
making a second hypercall which would give the same result.

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: <20190813113119.14804-23-anthony.perard@citrix.com>