OvmfPkg/QemuFwCfgLib: Implement SEV internal function for Dxe phase

OvmfPkg/QemuFwCfgLib: Implement SEV internal function for Dxe phase

When SEV is enabled, the DMA must be performed on unencrypted pages.
So when get asked to perfom FWCFG DMA read or write, we allocate a
intermediate (bounce buffer) unencrypted buffer and use this buffer
for DMA read or write.

Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>