OvmfPkg/XenBusDxe: In XenStore, replace type of Len from UINTN to UINT32.

Since a message to XenStore have a lenght of type UINT32, have
XenStore.c deal only with UINT32 instead of a mixmatch with UINTN.

This patch replaces the type of Len in WRITE_REQUEST and the type of the
argument Len of XenStoreWriteStore and XenStoreReadStore.

This patch should avoid to have type cast were it does not make sense to
have them.

Contributed-under: TianoCore Contribution Agreement 1.0

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>

Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

Build-tested-by: Scott Duplichan <scott@notabs.org>

Reviewed-by: Laszlo Ersek <lersek@redhat.com>

git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@16390 6f19259b-4bc3-4df7-8a09-765794883524