lib: Fix a typo and add documentation comments

Fix a typo in lib/alloc_phys.h and add documentation comments to all
functions in lib/vmalloc.h

Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Message-Id: <20200622162141.279716-6-imbrenda@linux.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>