lib: use PAGE_ALIGN

Since now PAGE_ALIGN is available in all architectures, start using it in
common code to improve readability.

Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Message-Id: <20200622162141.279716-4-imbrenda@linux.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>